It is demonstrated that the lowest energy path for the formation of a polyatomic molecule (applied to the HC-O formation) is easily calculated via a geometry-independent tight binding Hamiltonian fitted to accurate ab-initio configuration interaction (CI) total energies. This Hamiltonian not only reproduces the CI calculations accurately and efficiently, but also effectively identifies any CI energies happening to erroneously converge to excited states.

The aim of this research is to show that the processes of absorption charge exchange and photoassociation in A + B+ collisions together with the processes of AB(+) photodissociation in the case of strongly non-symmetric ion-atom systems, significantly influence the opacity of stellar atmospheres in ultraviolet (UV) and extreme UV (EUV) region. In this work, the significance of such processes for solar atmosphere is studied. In the case of the solar atmosphere the absorption processes with A = H and B = Mg and Si are treated as dominant ones, but the cases A = H and B = Al and A = He and B = H are also taken into consideration. The choice of just these species is caused by the fact that, of the species relevant for the used solar atmosphere model, it was only for them that we could determine the necessary characteristics of the corresponding molecular ions, i.e. the molecular potential curves and dipole matrix elements. It is shown that the efficiency of the examined non-symmetric processes within the rather wide corresponding quasi-molecular absorption bands in the far-UV and EUV regions is comparable and sometimes even greater than the intensity of the known symmetric ion-atom absorption processes, which are included now in the models of the solar atmosphere. Consequently, the presented results suggest that the non-symmetric ion-atom absorption processes also have to be included ab initio in the corresponding models of the stellar atmospheres.

The aim of this research is to show that the radiative processes in strongly non- symmetric ion- atom collisions significantly influence on the opacity of the solar photosphere in UV region. Within this work only the He+H+ and H+A(+) ion-atom systems, where A is the atom of one of the metal (Mg, Si and Al), are taken in to account. It is caused by the fact that the needed characteristics of the corresponding molecular ions, i.e. molecular potential curves and dipole matrix elements, have been determined by now. Here the non- symmetric radiative processes are considered under the conditions characterizing the non-LTE standard model of the solar atmosphere (Vernazza J, Avrett E and Loser R 1981 ApJS 45 635), which gives the possibility to perform all needed calculations and determined the corresponding spectral absorption coefficients. It is shown that the examined processes generate rather wide quasi-molecular absorption bands in the UV and VUV regions, whose intensity is comparable and sometimes even larger than the intensity of known one's caused by the H+H+ radiative collision processes, which are included now in the solar atmosphere models. Consequently, the presented results suggest that the non-symmetric ion- atom absorption processes have to be also included in standard models of the solar atmosphere.

Employing both multireference configuration interaction (MRCI) and density functional theory (DFT) methods, we have studied the interaction of O₂ with a tetrahedral Al₄ cluster in the total spin triplet state. For a parallel to the base approach of O₂ facing an apex of the pyramid, the O₂ adsorption is hindered by a barrier. Both the MRCI and the DFT calculations show that after a small barrier, there are two local energy minima: a shallow one just above the apex atom and another deeper one below the apex atom. The latter corresponds to dissociative O₂ adsorption. We discuss the implications of these findings for the understanding of O₂ adsorption on defect sites of Al surfaces.

The main aim of this work is to estimate the total contribution of the processes of He-2(+) molecular ion photodissociation and He + He+ collisional absorption charge exchange to the opacity of DB white dwarf atmospheres, and compare this with the contribution of He- and other relevant radiative absorption processes included in standard models.The method for the calculations of the molecular ion He+ 2 photodissociation cross-sections is based on the dipole approximation and quantum-mechanical treatment of the internuclear motion, while the quasi-classical method for describing absorption processes in He + He+ collisions is based on the quasi-static approximation.Absorption coefficients are calculated in the region 50 nm <= lambda <= 850 nm and compared with the corresponding coefficients of other relevant absorption processes; the calculations of the optical depth of the atmosphere layers considered are performed in the far-UV and VUV regions; the contribution of the relevant absorption processes to the opacity of DB white dwarf atmospheres is examined.We examined the spectral ranges in which the total He-2(+) and He- absorption processes dominate in particular layers of DB white dwarf atmospheres. In addition, we show that in the region of lambda less than or similar to 70 nm the process of H(1s) atom photoionization is also important, in spite of the fact that the ratio of hydrogen and helium abundances in the DB white dwarf atmosphere considered is 1:10(5).

We have studied the interaction of an oxygen molecule with Al clusters and Al(111) using both wave-function-based quantum chemistry methods and density functional theory (DFT). These calculations were motivated by the fact that molecular beam experiments indicate that the adsorption Of O-2 on Al(111) should be activated whereas periodic DFT calculations yield purely attractive adsorption paths for almost all impact configurations Of O-2 on Al(111). On small Al-4 clusters, accurate wave-function-based quantum chemistry methods find a non-vanishing barrier in the O-2 adsorption. The DFT calculations for slabs and larger Al clusters confirm the important role of spin effects for the O-2 dissociation barrier on Al. The results indicate that exchange-correlation effects play a crucial role for the determination of the adsorption barrier in the O-2/Al system but their determination is hampered by serious technical problems that are discussed in detail.

We investigated the feasibility of formation of the C2H3O+ chemdon during collisions of O(P-3) with the CH3C ((a) over tildeA(2)),CH2CH ((a) over tildeA"), CH3C (X(2)A"), and CH2CH (X(2)A') radicals, which are possible intermediates in the oxidation of 2-butyne. We determined the C-O bond dissociation energies and the ionization energies of the neutral C,H,O species as well as the C-O bond dissociation energies of C2H3O+. We conclude that the CH3C((a) over tilde (4)A(2)) radical is the most probable precursor of the chemiion, in agreement with experimental evidence. (c) 2005 Wiley Periodicals, Inc.

It is demonstrated that the potential-energy surface and the lowest-energy path for a polyatomic molecule (applied to the CH+O system) is accurately calculated via bond-length-dependent tight-binding Hamiltonian, fitted to ab initio configuration-interaction (CI) total energies. This Hamiltonian not only reproduces the CI energies accurately and efficiently, but also effectively recognizes and identifies CI energy values that may erroneously converge to excited states. The resulting normal mode frequencies are in very good agreement with experiment.

It is demonstrated that the reaction path for a polyatomic molecule (applied to the HCO molecule) is easily calculated via ab-initio configuration interaction (CI) total energies. This Hamiltonian not only reproduces the CI calculations accurately and efficiently, but also effectively corrects any CI energies happening to erroneously converge to excited states.

We have investigated the vinoxy radical and its ion at the MCSCF level of accuracy. We have obtained their equilibrium geometries and their C–O bond energies, using analytic gradients for geometry optimizations. We have also obtained the adiabatic ionization energy of the vinoxy radical and have verified that this radical does not autoionize.

We have computed accurate potential energy curves of the ground states of the PO(X 2Π), PO+(X 1Σ+) and PO−(X 3Σ−) species by multireference configuration interaction and coupled-cluster methods and have obtained accurate spectroscopic constants for each species. We have also determined the effect of core on the properties above and have obtained the PO complete basis set limit by the multireference method for the equilibrium energy, bond distance, dissociation energy, harmonic frequency, and dipole moment.

We have verified theoretically the assumption advanced by experimental groups that the CH3C radical in its (a) over tilde (4)A(2) excited state is one of the possible precursors of the CH3CO+ ((X) over tilde (1)A(1)) chemi-ion generated in the oxidation of 2-butyne. At the same time we have showed that the ground state of the CH3C radical cannot be such a precursor because of the existence of a potential barrier along the reaction coordinate in combination with its fast isomerization to CH2CH. (C) 2003 Elsevier Science B.V. All rights reserved.

The astrophysical importance of the SiH radical has motivated significant experimental and theoretical work. However, only the X 2Π and A 2Δ states of SiH have been extensively investigated experimentally, while the study of higher excited states is rather limited. From a theoretical point of view, most of the studies have been focused on spectroscopic and thermochemical quantities of the ground state. The lack of accurate spectroscopic parameters (re,De,ωe,ωexe,αe,D̄e,Te) pertaining to higher excited states was the driving force of the present work, in line with our previous study of the isovalent CH molecule [A. Kalemos, A. Mavridis, and A. Metropoulos, J. Chem. Phys. 111, 9536 (1999)]. Using the multireference configuration interaction approach coupled with very large correlation-consistent basis sets, we have constructed potential energy curves for 18 molecular states correlating to Si(3P,1D,1S,5S,3P,1P)+H(2S). At the same level, the potential energy curve of the ground SiH+ state (X 1Σ+) has also been constructed. We report total energies, dissociation energies, and the usual spectroscopic constants for Si28–1,2H and for all states studied. Most of our results are in excellent agreement with existing experimental values. In particular, we believe that our dissociation energy for the X state, De=73.28 kcal/mol, is the most reliable reported so far in the literature.

We have determined the favorable geometries and intermediate HCO states that allow the chemi-ionization reaction O(3P)+CH→HCO+(X 1Σ+)+e− to take place in low-energy collisions, when CH is either in the ground X 2Π or in the first excited a 4Σ− state.

Using the Golden Rule approximation, the predissociation lifetime of the v=0 vibrational level of the CH (E2Π) state, due to its strong avoided crossing with the CH (F2Π) state, was found to be of the order of τpre≈2 ps. This agrees well with the recent rotationally resolved experimental lifetimes. As expected, higher vibrational levels of the E and F states are extremely short-lived, having a τpre of the order of a few fs.

With the high accuracy afforded by the sextuple correlation consistent basis set of Dunning, we have calculated energy levels, dissociation energies, equilibrium distances, and other spectroscopic constants for eleven valence and four Rydberg states of the CH radical. Comparisons with experimental and previous theoretical results are made for each state that has been treated. An understanding of their binding is attempted by means of simple valence bond–Lewis diagrams.

Using a variety of ab initio methods, SCF, CISD, MP2, MP4, MCSCF, and MCSCF+1+2, we have studied the electronic and geometical structure of the ground and low-lying states of the isovalent radicals NCl2, PCl2 and NCl, PCl. In particular, we have examined the states ($) over tilde X(2)B(1), ($) over tilde A(2)A(1), ($) over tilde (BB2)-B-2, ($) over tilde C(2)A(1), ($) over tilde D(2)A(2) and ($) over tilde X(2)B(1), ($) over tilde A(2)A(1), ($) over tilde (BB2)-B-2, ($) over tilde C(2)A(1), ($) over tilde (DB2)-B-2, ($) over tilde E(2)A(2) for NCl2 and PCl2, respectively. These calculated manifolds can be proved useful in elucidating the complicated electronic spectroscopy of the triatomic species. For the diatomic molecules we have examined the states X(3) Sigma(-), a(1) Delta, and b(1) Sigma(+), while, for the ground X(3) Sigma(-) state only, we constructed full energy potential curves in an effort to obtain accurate dissociation energies.

We have obtained by ab initio calculations the energetics and the geometries of the X(1) Sigma(+), a(3) Pi, and A(1) Pi valence states of the ScPH+ cation, which we have found to be nonrigid in its ground state. We have also examined its bond energies and lengths vis-a-vis the same quantities of the free ScP+ and PH species and have generated potential curves for its Sc+ + PH and ScP+ + H asymptotic channels.