Dalton is a powerful general-purpose program system for the study of molecular electronic structure at the Hartree-Fock, Kohn-Sham, multiconfigurational self-consistent-field, Møller-Plesset, configuration-interaction, and coupled-cluster levels of theory. Apart from the total energy, a wide variety of molecular properties may be calculated using these electronic-structure models. Molecular gradients and Hessians are available for geometry optimizations, molecular dynamics, and vibrational studies, whereas magnetic resonance and optical activity can be studied in a gauge-origin-invariant manner. Frequency-dependent molecular properties can be calculated using linear, quadratic, and cubic response theory. A large number of singlet and triplet perturbation operators are available for the study of one-, two-, and three-photon processes. Environmental effects may be included using various dielectric-medium and quantum-mechanics/molecular-mechanics models. Large molecules may be studied using linear-scaling and massively parallel algorithms. Dalton is distributed at no cost from http://www.daltonprogram.org for a number of UNIX platforms.

This paper presents the coupled cluster/molecular mechanics (CC/MM) and self-consistent field/molecular mechanics (SCF/MM) approaches for wavefunctions, energies and response properties. Two physically different theories are derived, the mean-field and the direct-field interaction approaches, together with expressions for the optimization condition of both variational and non-variational wavefunctions and energies. Also derived are the linear response functions at the CC/MM and SCF/MM levels of theory, and the expressions are compared with the vacuum response functions.

We present a combined quantum mechanics/molecular mechanics and quantum statistical investigation of the interactions between a molecule (SO2) and an aerosol particle including rate constants for the uptake process. A coupled cluster/molecular mechanics method including explicit polarization is used along with a quantum statistical method for calculating sticking coefficients. The importance of the polarization of the classical subsystem (the aerosol particle), the size of the classical subsystem and the size of one-electron basis sets are studied. The interaction energy is divided into van der Waals, electrostatic and polarization contributions. Relevant binding sites for the evaluation of the sticking coefficient are identified. These are classified into three groups according to the strength of the molecule–aerosol particle interaction energy. The identification of binding sites provides relevant information used in the quantum statistical method and thereby knowledge of the magnitude of the sticking coefficients for the different binding sites along with the total rates for the uptake processes between the aerosol particle and the SO2 molecule.

Liquid water is investigated theoretically using combined molecular dynamics (MD) simulations and accurate electronic structure methods. The statistical mechanically averaged molecular properties of liquid water are calculated using the combined coupled cluster/molecular mechanics (CC/MM) method for a large number of configurations generated from MD simulations. The method includes electron correlation effects at the coupled cluster singles and doubles level and the use of a large correlation consistent basis set. A polarizable force field has been used for the molecular dynamics part in both the CC/MM method and in the MD simulation. We describe how the methodology can be optimized with respect to computational costs while maintaining the quality of the results. Using the optimized method we study the energetic properties including the heat of vaporization and electronic excitation energies as well as electric dipole and quadrupole moments, the frequency dependent electric (dipole) polarizability, and electric-field-induced second harmonic generation first and second hyperpolarizabilities. Comparisons with experiments are performed where reliable data are available. Furthermore, we discuss the important issue on how to compare the calculated microscopic nonlocal properties to the experimental macroscopic measurements.

This chapter concerns highly accurate methods for predicting molecular properties of molecules in solution. Our emphasize is on solvent effects or the description of a localized molecular fragment within a larger molecular system, where we solve the problem at hand by considering the effect of a medium, e. g. the solvent or the surrounding molecular system, on the relevant part of the system. Here, we refer to the fact that most chemistry in larger molecular systems may be located at a specific region, while the rest of the molecule is almost unchanged. For example, in the case of a well localized electronic transition in a solute molecule, the solvent molecules may be seen as secondary molecules, since they are not directly involved in the physical process. Here, we will focus on the molecular properties of liquid water. We present and review the theory and applications of two coupled cluster methodologies: (I) the coupled cluster/dielectric continuum model (CC/DC) and (II) the coupled cluster/dielectric continuum model (CC/DC).

We present a theoretical study of the vertical electronic excitation energies of the alanine zwitterion in an aqueous environment. The study is based on calculations using the combined coupled cluster/molecular mechanics (CC/MM) method. The CC/MM results (using different water potentials) are compared to calculations based on density functional theory (DFT) using the B3LYP functional. Special attention is payed to the lowest nO→πCO∗ transition and higher lying states including the πCO→πCO∗. We find that the spectra obtained using CC/MM are in good agreement with experiments. On the other hand, less satisfactory results are found for the spectrum calculated using DFT.

We present a study of the interaction between a phenol molecule and an aerosol particle. The aerosol particle is represented by a cluster of 128 water molecules. Using a classical approach, we present interaction energy surfaces for different relative distances and for three orientations of phenol relative to the particle. From the energy surfaces we find the reaction pathways with the largest interaction between the molecule and the particle. We use a quantum mechanics/molecular mechanics (QM/MM) method to calculate a potential energy curve for each reaction path. Coupled cluster methods are used for the part of the system described by quantum mechanics, while the part described by molecular mechanics is represented by a polarizable force field. We compare results obtained from the classical approach with the QM/MM results. Furthermore, we use the QM/MM results to calculate mass accommodation coefficients using a quantum-statistical (QM-ST) model and show how the mass accommodation coefficient depends on the relative orientation of phenol with respect to the aerosol particle.

A theoretical study on the origin of the common electronic excitations in amino acids is presented, focusing on the excited states of glycine, alanine and the related substructures formic acid, acetic acid, propionic acid, ammonia, methylamine, and ethylamine. Special attention is given to the valence excitation from the nonbonding lone-pair on the carboxylic oxygen atom to the antibonding pi-orbital (n(O) --> pi*(CO)) and the first Rydberg excitation from the nonbonding lone-pair on the nitrogen atom (n(N) --> 3s). From extensive calculations on formic acid and methylamine, different basis sets and electron correlation treatments are benchmarked using a hierarchy of coupled cluster (CC) methods, consisting of CCS, CC2, CCSD, CCSDR(3), and CC3, in combination with augmented correlation consistent basis sets. The dependence of the excitation energies on the size of the backbone structure in the two groups of molecules is investigated, and 0-0 transition energies for the n(O) --> pi*(CO) and n(N) --> 3s transitions are calculated for the smallest molecules. Excellent agreement with experimental values is found where secure experimental assignments are available. A few outstanding problems in the experimental assignments found in the literature are described for both the carboxylic acids and the amines. Final predictions for vertical excitation energies are given for all molecules, including glycine and alanine where no gas-phase experimental results are available. Finally, calculations on protonated amino acids are presented showing an isolation of the n(O) --> pi*(CO) from higher lying states by as much as 1.9 eV for alanine.

The combined linear response coupled cluster/molecular mechanics (CC/MM) scheme including mutual polarization effects in the coupling Hamiltonian is applied together with supermolecular CC methods to the study of the gas-to-aqueous solution blue shift of the n --> pi* excitation energy in acetone. The aug-cc-pVDZ basis set is found to be adequate for the calculation of this excitation energy. In the condensed phase, the shift in the excitation energy is obtained by statistical averaging over 800 solute-solvent configurations extracted from a molecular dynamics simulation. We find the shift to be around 1100-1200 cm(-1) depending on the specific model used to describe solvent polarization. The importance of including explicit polarization in both the molecular dynamics simulation as well as the CC/MM calculations is emphasized. Furthermore, the significant dependence of the excitation energy on the CO bond length of acetone is discussed.

In this article we report the first calculations of second harmonic generation second hyperpolarizability of liquid water using coupled cluster/molecular mechanics (CC/MM) methods or coupled cluster/dielectric continuum (CC/DC) methods. The latter approach treats the solvent as an isotropic homogeneous fluid while the former accounts for the discrete nature of the solvent molecules. The CC/MM approach may include or exclude polarization effects explicitly. Alternatively, polarization effects may be included using perturbation theory. The CC descriptions implemented are the coupled cluster second-order approximate singles and doubles (CC2) and coupled cluster singles and doubles models. The second harmonic generation second hyperpolarizabilities are, depending on the model, obtained using either an analytical implementation of the cubic response function or using an analytical implementation of the quadratic response function combined with the finite field technique. The CC/MM results for the second harmonic generation second hyperpolarizability compare excellently with experimental data while a significant overestimation is found when using the CC/DC model. Particular, the cavity radius in the CC/DC calculations have an enormous effects on this fourth-order property.

We present a study of the blueshift of the n-->pi* electronic transition in formaldehyde in aqueous solution using a combined coupled cluster/molecular mechanics model including mutual polarization effects in the Hamiltonian. In addition, we report ground and excited state dipole moments. Configurations are generated from molecular dynamics simulations with two different force fields, one with and one without an explicit polarization contribution. A statistical analysis using 1200 configurations is presented. Effects of explicit polarization contributions are found to be significant. It is found that the main difference in the effects on the excitation energies arises from the fact that the two force fields result in different liquid structures, and thus a different set of configurations is generated for the coupled cluster/molecular mechanics calculations.

In this article we present the first theoretical study of solvent effects on the rotatory strength tensor. The system chosen is solvated formaldehyde for which only one tensor element is nonvanishing, and the solvent is modeled as a linear, homogeneous, and isotropic dielectric continuum. We present results using both an equilibrium and a nonequilibrium description of the solvent. Four illustrative solvents (ethyl ether, acetone, methanol, and water) are considered together with the corresponding results for formaldehyde in vacuum. We utilize the following ab initio methods: the coupled cluster model including singles and doubles (CCSD) and the coupled cluster second-order approximate singles and doubles (M). Furthermore, we compare the coupled cluster results with the corresponding uncorrelated self-consistent-field (SCF) results. In addition to the rotatory strength tensor we also present solvent effects on the low-lying electronic excitation energies and corresponding ordinary intensities using both the length and velocity gauges. We find that both correlation and solvent effects have a significant influence on the transition properties. The introduction of the solvent is, in some cases, found to result in a sign change of the rotatory strength tensor elements which clearly demonstrates the importance of a proper description of the solvent influence on this property.

In this paper we present linear response properties of liquid water calculated using the second-order approximate coupled cluster singles and doubles (CC2) and the coupled cluster singles and doubles (CCSD) wave function parametrizations combined with different molecular mechanics models. We discuss different approaches within the QM/MM scheme where the solvent molecules are represented by point charges and induced dipole moments. Here, we address two important aspects in QM/MM methods. First, we aim at obtaining MM parameters from an iterative self-consistent approach. Second, we show how to reduce the computational costs of the models considerably without reducing the quality of the results. Excitation energies, transition moments, and oscillator strengths are compared within the different approaches. Finally, we report the CC2 frequency dependent polarizability compared to the CCSD results.

We present an ab initio study of the n→π∗ electronic excitation energy together with the corresponding oscillator strength and rotatory strength tensor for two optically active cyclopropanone related structures: S,S-dimethylcyclopropanone and R,R-t-butylcyclopropanone. We discuss and compare the available experimental data for the n→π∗ electronic excitation energy and rotatory strength with the theoretical calculations.

We present ab initio calculations of the optical rotation of S-propylene oxide in both gas phase and solution using the coupled cluster methodology combined with a dielectric continuum description of the solvent. The coupled cluster calculations are performed using the CCS, CC2, CCSD and CC3 methods. None of the presented gas phase results are in accord with the experimental sign of the optical rotation at 355 nm. Thereby, the experimental sign change between the gas phase and the cyclohexane solution optical rotation at 355 nm is not reproduced theoretically. The vibrational effects are considered to be significant and may be of crucial importance in order to bring accordance between the calculated and the experimentally established sign of the gas phase optical rotation at 355 nm.

In this article, we study the n - pi* electronic transition in aqueous microsolvated formaldehyde using the coupled cluster (CC) and coupled cluster/molecular mechanics (CC/MM) methods. The CC models used are the coupled cluster singles and doubles (CCSD) and the coupled cluster second-order approximate singles and doubles (CC2) methods. The CC/MM model includes electrostatic and mutual polarization effects on the calculated electronic excitation energies. The CC/MM shifts of the lowest electronic excitation energy compare successfully to the corresponding shifts as defined in the supermolecular approach. Finally, we include, in addition to the explicit water molecules in the supermolecular calculations, a dielectric medium to account for the long-range interactions. The result for the shift in electronic excitation energy compares well with both other theoretical approaches and available experimental data.

In this paper we employ a recently developed method which combines coupled cluster and molecular mechanics (CC/MM) theories to study the effects of solvation on some molecular properties of liquid water.We focus on the effect of varying the parameters used to define the classically treated molecules, i.e. partial charges and point dipole polarizabilities. Furthermore, we define a set of ‘polarization charges’ which include the effects of a polarizable environment (the MM system) and correctly reproduces the ground state energy of the total system. However, we find that for these polarization charges that specific ground and excited state molecular properties are not reproduced correctly. Specially, the polarizability is very difficult to model using a simple point charge model with charges confined to the MM atoms. Furthermore, we study some excited state properties such as electronic excitation energies and corresponding dipole transition moments of liquid water.Finally, we explore the effect of introducing few or many classically treated molecules in the CC/MM calculations.