Investigations on how molecular bridge structure influences electronic communication between functional units provide key insights into structure-property relationships and are essential for designing materials with tailored electronic properties. Here, we examine how the bridge electron density affects magnetic exchange (2J) across a series of diradicals and charge transfer (CT) coupling in their corresponding mixed-valence (MV) anions and cations. Four diradicals composed of polychlorinated pyridyldiphenylmethyl (PyBTM) units linked by substituted 2,7-fluorene bridges of varying electron density were synthesized and electrochemically converted into isostructural MV species, granting access to magnetic and CT coupling across three redox states within an identical atomic framework. For the diradicals, combined experimental and quantum chemical studies showed stronger antiferromagnetic coupling with higher bridge electron density. Analysis of direct and kinetic exchange contributions revealed dynamic spin polarization as the main factor governing magnetic coupling with electron-rich bridges yielding larger |2J| values. Complementary analyses of the MV anions and cations within the generalized Mulliken-Hush framework demonstrated effective CT coupling in both redox states. In each case, an energetically elevated CT state with bridge-centered charge localization strongly influenced the coupling. Frontier orbital analysis of hole and electron transfer clarified how donor- and acceptor-substitution modulates CT coupling. These results establish bridge electron density as a key parameter for tuning magnetic exchange and CT coupling, providing a basis for designing molecular architectures with controllable charge transport and magnetic properties.

Achieving quantitative insight into the noncovalent interactions that govern molecular function and biological activity in complex assemblies remains a major challenge for quantum chemical analysis, particularly in systems such as nucleic acids where standard force-fields are known to struggle. Local energy decomposition (LED) provides gold-standard coupled-cluster descriptions of intermolecular interactions but can become difficult to interpret chemically when interacting fragments are covalently connected, such as ligands embedded in nucleic acid frameworks or functional groups linked through bonding networks. In these situations, shared electron density across fragment boundaries obscures the physical interpretation of energy contributions. Here we introduce CovaLED, an extension of the LED scheme that enables a rigorous treatment of covalent connectivity within the LED framework. Application to nucleic-acid-based recognition systems demonstrates the capabilities of the approach. In a riboswitch RNA-ligand complex, CovaLED reveals how methylation of guanine leads to a loss of stabilizing hydrogen-bonding interactions, consistent with experimental binding affinity trends. In a segment of human DNA, the method enables accurate quantification of interactions between nucleotides covalently linked within the backbone. CovaLED thus enables coupled-cluster-level energy decomposition in realistic biomolecular systems, where covalent structure and noncovalent recognition are intrinsically intertwined.

N-anchored tripodal N-heterocyclic carbene ligands, satTIMMNMes and Me2TIMMNMes, were synthesized and used to stabilize a series of mid-valent Fe(IV) and high-valent Fe(V) and Fe(VI) nitrides. The Fe(IV) and Fe(V) species adopt trigonal-pyramidal and trigonal-bipyramidal geometries, respectively, whereas the Fe(VI) nitrides exhibit octahedral coordination, representing rare examples of structurally characterized iron(VI) nitrido complexes. All complexes were characterized by single-crystal X-ray diffraction, multinuclear (1H, 13C, 15N, and 19F) NMR, zero- and applied-field 57Fe Mössbauer, electron paramagnetic resonance, as well as vibrational and electronic absorption spectroscopy. Combined spectroscopic, electrochemical, and computational studies examined how systematic variation in the NHC backbone modulates the electronic structures of the [Fe≡N]n+ and Fe-CNHC moieties. Structural and spectroscopic parameters of Me2TIMMNMes-based Fe(IV-VI) nitrides closely resemble those of parent TIMMNMes analogues, while more pronounced deviations are observed for satTIMMNMes derivatives. 57Fe Mössbauer spectroscopy at 80 K revealed an unusually negative isomer shift for the tetravalent (d4, S = 0) [(satTIMMNMes)FeIV≡N]+ (δ = -0.45 mm s-1), distinct from Me2TIMMNMes and TIMMNMes analogues (δ = -0.36 and -0.35 mm s-1), attributable to enhanced Fe-CNHC covalency in satTIMMNMes ligand. Cyclic voltammetry, supported by theoretical calculations, quantifies relative ligand π-donation in the Fe(IV) nitrido complexes, complementary to established NHC σ-donor descriptors, yielding the trend satTIMMNMes < TIMMNMes < Me2TIMMNMes. Consistent with this trend, structural and EPR data indicate differing degrees of Jahn-Teller distortion among the paramagnetic Fe(V) nitrides (d3, S = 1/2). In contrast, the structural and spectroscopic differences are relatively diminished in highly covalent Fe(VI) nitrides (d2, S = 0).

In light of major developments over the past decades in both quantum computing and simulations on classical hardware, it is a serious challenge to identify a real-world problem where quantum advantage is expected to appear. In quantum chemistry, electronic structure calculations of strongly correlated, i.e. multi-reference problems, are often argued to fall into such category because of their intractability with standard methods based on mean-field theory. Therefore, providing state-of-the-art benchmark data by classical algorithms is necessary to make a decisive conclusion when such competing development directions are compared. We report cutting-edge performance results together with high accuracy ground state energy for the Fe_4S_4 molecular cluster on a CAS(54,36) model space, a problem that has been included quite recently among the list of systems in the Quantum Advantage Tracker webpage maintained by IBM and RIKEN. Pushing the limits even further, we also present CAS-SCF based orbital optimizations for unprecedented CAS sizes of up to 89 electrons in 102 orbitals [CAS(89,102)] for the Fe_5S_12H_4^5- molecular system comprising twenty five open shell orbitals in its sextet ground state and an active spaces size of 331 electrons in 451 orbitals. We have achieved our results via mixed-precision spin-adapted ab initio Density Matrix Renormalization Group (DMRG) electronic structure calculations interfaced with the ORCA program package and utilizing the NVIDIA Blackwell graphics processing unit (GPU) platform. We argue that DMRG benchmark data should be taken as a classical reference when quantum advantage is reported. In addition, full exploitation of classical hardware should also be considered since even the most advanced DMRG implementations are still in a premature stage regarding utilization of all the benefits of GPU technology.

In this study, we report an analytical derivative implementation of static polarizability gradients, needed for the prediction of Raman spectra, in the ORCA program. Our implementation is capable of handling density functionals up to the meta-GGA level and makes use of RIJ and COSX integral approximations to improve the scaling of calculations using both pure and hybrid functionals. Solvent effects can be included with CPCM. We examine the errors in RIJ and COSX Raman activities and find both approximations safe to use in routine calculations. In an effort to further systematically reduce errors resulting from RIJ, we introduce an automatic procedure to minimally augment auxiliary basis sets with diffuse functions if these are detected in the orbital basis set. Compared with numerical derivatives previously available in ORCA, we achieve speedups of up to 1-2 orders of magnitude. Finally, we demonstrate that these developments allow us to calculate fully analytical harmonic Raman spectra of systems containing more than 100 atoms, with the polarizability gradient only introducing a moderate overhead (lower than 10%) to the Hessian calculation.

openCOSMO-RS is an open-source predictive thermodynamic model that can be applied to a broad range of systems in various chemical and biochemical engineering domains. This study focuses on improving openCOSMO-RS by introducing a new dispersion term based on atomic polarizabilities. We evaluate different methods for processing polarizability data, including scaling and combining it to compute segment-segment dispersion interaction energies, with a focus on halocarbon systems. The results demonstrate that the modified model outperforms our previous method developed in the first part of this work (Grigorash et al., 2024) , while at the same time requiring fewer adjustable parameters. The approach was applied to a broad dataset of over 50,000 data points, consistently increasing the accuracy across a variety of data types. These findings suggest that atomic polarizability is a valuable descriptor for refining dispersion interactions in predictive thermodynamic models.

Oxidoiron(IV) species are crucial intermediates for the functionalization of C–H bonds by biological oxidases and oxygenases, and understanding the relations between ligation, electronic structure and reactivity of the FeIV=O unit is key to the rational design of bioinspired catalysts. Here, a series of FeIV=O complexes based on a macrocyclic tetracarbene ligand and with different anionic trans-axial ligands (Lax = CF3COO−, tBuS−, AdaS−) has been comprehensively characterized, including X-ray crystallography in two cases as well as zero-field and applied-field 57Fe Mössbauer spectroscopy, magnetic susceptibility measurements, magnetic circular dichroism (MCD), and helium tagging infrared photodissociation (IRPD). This establishes a detailed picture of the electronic structures of these ferryl complexes featuring a well separated S = 1 ground state, and the influence of the trans-ligand. The confinement to triplet-only pathways in C−H bond activations and comparison with the parent FeIV=O complex (Lax = MeCN) allows to uncover trends in hydrogen atom abstraction (HAA) ability without perturbations from different extents of two-state reactivity (TSR). Kinetic studies on HAA for C‒H substrates with BDEs up to 78 kcal mol−1 show relatively minor effects of the axial ligands, with rates following an electrophilc trend in the order Lax= MeCN > CF3COO–  RS–. This contrasts the anti-electrophilic trend found for a related series of tetramethylcyclam (TMC) based FeIV=O complexes for which TSR is dominant. For the present systems, temperature-dependent KIEs in combination with computations at the DFT and ab initio level of theories suggest a semiclassical model with modest tunneling contributions that are most pronounced in case of a strongly donating trans-thiolate. This study thus provides a combined experimental and computational basis for understanding intrinsic axial ligand effects and the effect of a trans-axial thiolate in S = 1 ferryl intermediates.

We report the synthesis and comprehensive characterization of a closed-shell iron(IV) oxo (ferryl) complex, S = 0, which is accessible through multiple methods and stabilized by a tripodal tris-N-heterocyclic carbene (NHC) ligand. Reaction of the FeI precursor [(Me2TIMMNMes)FeI(η1-N2)](PF6) (1) (Me2TIMMNMes = tris-[2-(3-mesityl-4,5-dimethyl-imidazolin-2-ylidene)methyl]amine) with an excess of N2O at -78 °C in THF yields the reactive FeIII oxo complex [(Me2TIMMNMes)FeIII(O)](PF6) (3). One-electron oxidation of the in situ generated 3 with [Cp2Fe][PF6] provides access to a rare, nonmagnetic (d 4, S = 0) FeIV oxo complex, namely [(Me2TIMMNMes)FeIV(O)](PF6)2 (4). Alternatively, tetravalent 4 could also be obtained by treatment of the divalent iron complex [(Me2TIMMNMes)FeII(THF)](PF6)2 (2-THF) with 1 equiv of trimethylamine N-oxide (TMAO) or by the photolysis reaction of the ferrous sulfoxide isotopomers [(Me2TIMMNMes)FeII(MeS(16/18O)Ph)](PF6)2 (2-16/18OSR2). Although the fleeting nature of trivalent oxide 3 precludes extensive characterization and isolation in the solid state, the one-electron oxidation product [(Me2TIMMNMes)FeIV(O)](PF6)2 (4) is isolable and was reproducibly synthesized as well as fully characterized, including CHN elemental analysis, multinuclear NMR, IR vibrational, UV/vis electronic absorption, zero- and applied-field 57Fe Mössbauer spectroscopy, and single-crystal X-ray crystallography studies. Diamagnetic 4 features a remarkably short Fe-O bond (d(Fe-O) = 1.576(1) Å) and a notably negative isomer shift (δ = -0.38 mm s-1) in the 57Fe Mössbauer spectrum. Computational analyses corroborate the + IV oxidation state and advocate an iron-oxygen triple bond, FeIV≡O. Preliminary reactivity studies show that the closed-shell iron(IV) oxo complex can mediate intermolecular oxygen-atom transfer chemistry. A reversible redox event at a half-wave potential, E1/2, of 1.25 V vs Fe(Cp)2/Fe(Cp)2+ in the cyclic voltammogram of 4 suggests the existence of an Fe(V) oxo species.

In this paper, an efficient implementation of the renormalized internally-contracted multreference coupled cluster with singles and doubles (RIC-MRCCSD) into the ORCA quantum chemistry program suite is reported. To this end, Evangelista's Wick d equation generator was combined with ORCA's native AGE code generator in order to implement the many-body residuals required for the RIC-MRCCSD method. Substantial efficiency gains are realized by deriving a spin-free formulation instead of the previously reported spin-orbital version developed by some of us. Since AGE produces parallelized code, the resulting implementation can directly be run in parallel with substantial speedups when executed on multiple cores. In terms of runtime, the cost of RIC-MRCCSD is shown to be between single-reference RHF-CCSD and UHF-CCSD, even when active space spaces as large as CAS(14,14) are considered. This achievement is largely due to the fact that no reduced density matrices (RDM) or cumulants higher than three-body enter the formalism. The scalability of the method to large systems is furthermore demonstrated by computing the ground-state of a vitamin B12 model comprised of an active space of CAS(12, 12) and 809 orbitals. In terms of accuracy, RIC-MRCCSD is carefully compared to second- and approximate fourth-order n-electron valence state perturbation theories (NEVPT2, NEVPT4(SD)), to the multireference zeroth-order coupled-electron pair approximation (CEPA(0)), as well as to the IC-MRCCSD from Kohn. In contrast to RIC-MRCCSD, the IC-MRCCSD equations are entirely derived by AGE using the conventional projection-based approach, which, however, leads to much higher algorithmic complexity than the former as well as the necessity to calculate up to the five-body RDMs. Remaining challenges such as the variation of the results with the flow, a free parameter that enters the RIC-MRCCSD theory, are discussed.

In exploring the toxicity of micro- and nanoplastics, molecular simulations of plastic nanoparticles have been gaining traction recently. Modeling of nanoplastics involves the folding of multiple polymeric chains into an entangled particle, which is a challenging process, necessitating thorough optimization of the folding procedure. In this contribution, we use the simulated annealing procedure in a systematic workflow for preparing stable nanoplastic structures for the first time, based on the CHARMM36 force field. On the structures prepared with the fine-tuned protocol, we carry out quantum chemical geometry optimizations with the GFN2-xTB method, followed by benchmarking single-point calculations with GGA and hybrid DFT functionals. We further demonstrate the applicability of this approach through four plastic systems, including polyethylene, polypropylene, polystyrene, and nylon-66. Remarkably, the geometry of the resulting most stable assemblies show similarities to features observed earlier theoretically and experimentally for such systems. For polyethylene, a highly ordered, crystalline structure is obtained, in which the polymer chains possess long sections with all C-C-C-C units in trans configuration. For polypropylene and polystyrene, helical structures are observed, formed by alternating gauche and trans configurations along the backbone. For nylon-66, the structure-directing effect of the hydrogen bonds between amide moieties complicates the folding, resulting in parallelly arranged hydrogen bonding chains throughout the particle. Especially, we make the whole set of optimized structures available for the community in an online repository, with hopes of advancing simulation studies in the field, even making ensemble simulations accessible.

The cluster-in-molecule (CIM) local correlation approach with an accurate distant pair correlation energy correction is presented. For large systems, the inclusion of distant pair correlation energies is essential for the accurate prediction of absolute correlation energies and relative energies. Here, we propose a simple and efficient scheme for evaluating the distant pair correlation energy correction for the CIM approaches. The corrections can be readily extracted from electron correlation calculations of clusters with almost no additional effort. Benchmark calculations show that the improved CIM approach can recover more than 99.94% of the correlation energy calculated by the parent method. By combining the CIM approach with the domain-based local pair natural orbital (DLPNO) local correlation approach, we have provided accurate binding energies at the CIM-DLPNO-CCSD(T) level for a test set consisting of eight weakly bound complexes ranging in size from 200 to 1027 atoms. With these results as the reference data, the accuracy and applicability of other electron correlation methods and a few density functional methods for large systems have been assessed.

This paper reports the improvement in the efficiency of embedded-cluster model (ECM) calculations in ORCA thanks to the implementation of the fast multipole method. Our implementation is based on state-of-the-art algorithms and revisits certain aspects, such as efficiently and accurately handling the extent of atomic orbital shell pairs. This enables us to decompose near-field and far-field terms in what we believe is a simple and effective manner. The main result of this work is an acceleration of the evaluation of electrostatic potential integrals by at least one order of magnitude, and up to two orders of magnitude, while maintaining excellent accuracy (always better than the chemical accuracy of 1 kcal/mol). Moreover, the implementation is versatile enough to be used with molecular systems through QM/MM approaches. The code has been fully parallelized and is available in ORCA 6.0.

In this work, a systematic computational investigation of the optical band gap (BG) problem of Co3O4 is carried out on the basis of the embedded cluster approach in combination with a series of particle/hole and wavefunction-based approaches. A total number of three experimental band gap energies for the bulk Co3O4 have been reported in the literature, the nature of which have remained controversial. This work will show that accurately describing the excited states and rationalizing these experimental band gaps require explicit treatment and analysis of strong electron correlation effects. These correlation effects enable low-energy optical excitations to emerge from both 'neutral' and 'ionic' antiferromagnetic configurations, depending on how the electronic structure reorganizes across the coupled high-spin tetrahedral Co(ii) (site A) and low-spin octahedral Co(iii) (site B) centers. To disentangle the contributions from these two distinct sites, this work introduces reference systems, Al2Co(ii)O4 and Co(iii)2ZnO4, which isolate the Co(ii) and Co(ii) sites, respectively. Tackling such a complex excited state problem requires going beyond density functional theory (DFT) particle/hole approaches and employing a range of single and multi-reference wavefunction based methods. In particular, complete active space configuration interaction self-consistent field (CASSCF) and its approximate CI variants in conjunction with 2nd order N-electron valence perturbation theory (NEVPT2) provide access to an accurate prediction of all three experimentally observed BG energies in Co3O4. Our calculations are consistent with the notion that the lowest energy band gap corresponds to the ligand field (LF) type of transitions within the local tetrahedral Co(ii) centers. Furthermore, the calculations predict that the middle energy band gap is a mixture of LF transitions at site A and metal-to-metal charge transfer (MMCT) transition across A-A ' and A-B/B-A ' pairs. These transitions give rise to Co(i) and Co(iii) configurations at site A, deviating from the original Co(ii) based configurations. This intermediate band is assigned to the actual experimentally observed optical band gap of Co3O4. Finally, the highest energy band gap is again a mixture of LF transitions at site A and ligand-to-metal charge transfer (LMCT), involving O 2p -> Co(ii)-3d transitions, with our calculations also indicating some contributions from other MMCT states. Hence, this later energy band corresponds to the actual semiconducting band gap that defines the semiconductor properties of Co3O4.

In this work, we assess the accuracy of the approximate fourth-order N-electron valence perturbation theory (NEVPT4(SD)) methodology for computing excited states of organic molecules. The well-established Thiel benchmark set was employed, comprising 225 vertical excitations spanning π → π*, n → π*, and σ → π* transition types. A state-specific canonicalization procedure was applied, enabling a direct comparison with CC3 reference data reported by Schreiber et al. J. Chem. Phys., 2008, 128, 134110. For both singlet and triplet excitations, NEVPT4(SD) systematically outperforms lower-order NEVPT variants, as well as previously reported complete active space second-order perturbation theory (CASPT2) results. A detailed analysis of the singlet excitations reveals that n → π* transitions have a slight tendency to be overestimated (by about 0.1 eV), while π → π* excitations tend to be slightly underestimated (by -0.04 eV). While this shift persists across all NEVPT perturbation orders, its magnitude decreases with higher-order treatments. Across the entire test set, NEVPT4(SD) has a very narrow error distribution with a peak very close to 0. Thus, this study demonstrates the robustness and high accuracy of NEVPT4(SD) for vertical excitation energies, highlighting its clear advantages over lower-order perturbative approaches while remaining computationally much more affordable than other multireference correlation approaches that proceed beyond second-order perturbation theory.

The accurate computation of high-spin/low-spin gaps remains a challenging task in computational chemistry, with significant implications for both theoretical studies and experimental applications. In this work, we present an exchange-dedicated perturbation theory (EDPT2) that allows an efficient calculation of exchange couplings in magnetic systems. Our approach builds on a previously developed second-order perturbative scheme based on de Loth's formalism but refines the treatment of singlet wave functions by explicitly incorporating ionic determinants in the zeroth-order description. The EDPT2 method is derived from a two-electron-two-center model and can be applied to multispin systems using minimal CAS-generated orbitals. A key advantage of EDPT2 lies in its computational efficiency, with a scaling of N4, where N is the number of basis functions. Benchmark calculations on diverse test systems demonstrate that EDPT2 achieves high-spin/low-spin gaps with accuracy comparable to the commonly used FIC-NEVPT2 method. Beyond its efficiency, EDPT2 provides valuable information on the mechanisms that govern magnetic exchange. The method allows for a detailed decomposition of second-order contributions, facilitating the identification of dominant exchange pathways. This is exemplified on two bis(nitronyl nitroxide) biradicals, where dynamic spin polarization emerges as the key exchange mechanism. Furthermore, using the example of a trisnitroxide triradical, we demonstrate how the insights from EDPT2 can be used to prepare selective multireference CI approaches. A combined DDCI1 approach with EDPT2-derived corrections is shown to successfully reproduce the experimental doublet-quartet gap.

In this work, the implementation of a partial fourth order N-electron-valence perturbation theory (NEVPT) is reported and numerically evaluated. The method, termed NEVPT4(SD), includes the internally contracted functions that span the first-order-interacting space (FOIS) and evaluates their contribution to second-order in the wave function and fourth order in the energy. The triple- and quadruple excitations that would additionally enter the second-order-interacting space (SOIS) are not included. As discussed by Grimme [Chem. Phys. Lett. 2001, 334, 99-106] in order to obtain a size-consistent method, it is necessary to also drop the fourth-order renormalization term if the quadruple excitations are dropped. The NEVPT4(SD) method is demonstrated to be perfectly size consistent. Computationally, the method is still fairly affordable and requires about the same time as a single iteration of the fully internally contracted (FIC) MRCI or MRCEPA(0) and significantly cheaper than the FIC MRCC that serves as the reference for our calculations. The accuracy tests show that NEVPT4(SD) offers significant accuracy improvements over NEVPT2 for transition metal atom/ion multiplets as well as diatomic bond breaking potential energy surfaces. We find that going to fourth order in perturbation theory essentially eliminates the need for a second d-shell, thus showing that the latter primarily serves to capture higher-order dynamic correlation effects that are not present in a second-order treatment. Although it captures fourth-order correlation effects, NEVPT4(SD) is numerically not a large improvement over NEVPT2 for the calculation of Heisenberg exchange couplings as illustrated by test calculations on Cu(II) dimers.

In this paper we present the theory and implementation of the spin-orbit coupling and Zeeman operators in the context of quasi-degenerate perturbation theory into the general spin restricted open-shell configuration interaction singles method. The implementation of the mentioned operators allows for the calculation of magnetic circularly polarized dichroism (MCD), L-edge X-ray absorption spectra (XAS) and X-ray magnetic circularly polarized dichroism (XMCD) spectra. The method was tested on calculating the MCD spectra of isostructural complexes [LCrIII(PyA)3NiII]2+, [LCrIII(PyA)3ZnII]2+and [LGaIII(PyA)3NiII]2+, with L = 1,4,7-trimethyl-1,4,7-triazacyclonanane and PyA- is the monoanion of pyridine-2-aldozime, where it correctly predicts the MCD signs of the lower optical transition of [LCrIII(PyA)3NiII]2+and [LGaIII(PyA)3NiII]2+. The capabilities of the method in computing L-edge XAS and XMCD spectra were tested on the model complexes [Cu(H2O)6]2+ and [Cu2(OAc)4(H2O)2], where it correctly calculates the L2,3-edge absorption and XMCD spectra, as well as on the antiferromagnetically coupled Cu-Fe dimer [(F8TPP)Fe(μ-O)Cu(TMPA)]+, where it correctly predicts the signs of the L2 and L3 edges of the Cu XMCD spectrum. To further illustrate the applicability of the method, the more complex L2,3-edge XAS and XMCD spectra of thiolate Fe complexes were also calculated.

This perspective offers a personal reflection on the evolution, current status, and open challenges of quantum chemistry in the context of large molecular systems. Beginning with Dirac’s famous 1929 prophecy, I revisit the historical trajectory of our discipline, from the development of early conceptual models to the emergence of density functional theory and correlated wavefunction methods. While enormous progress has been made in algorithmic sophistication, enabling accurate calculations on systems with thousands of atoms, I argue that accurate energies alone do not solve the entirety of chemical problems. Real-world applications demand more than electronic structure: solvation, entropy, conformational complexity, and the sheer complexity of large molecules challenge our methods and our assumptions. Equally important are the conceptual and philosophical tensions that continue to shape the field, especially the difficult balance between prediction and understanding. I advocate for a more integrative approach that values models, insight, and falsifiability alongside numerical precision. Finally, I briefly consider the emerging role of machine learning and the implications of automation for the future of theory. This article is not a review but a dialogue – with the field, with its history, and with its practitioners.

Spin-adapted configuration state functions (CSFs) provide a compact many-electron basis for open-shell molecules. This basis is employed in one flavor of the recently introduced iterative configuration expansion (ICE) selected CI method. In this work, we implemented spin-dependent operators like spin-orbit coupling and direct spin-spin coupling for use in quasidegenerate perturbation theory on top of nonrelativistic/scalar-relativistic ICE wave functions. At the core of the new implementation are matrix elements of spin tensor excitation operators between CSFs, which are evaluated as products of orbital-specific factors. Two applications, the electron paramagnetic resonance g-factors of a MoIII-based catalytic intermediate and the zero-field splitting in dioxygen, illustrate the capabilities of the new method.