Rafał R. Toczyłowski

Ground state potential energy curves for He2, Ne2, Ar2, He–Ne, He–Ar, and Ne–Ar: A coupled-cluster study

Potential energy curves for three homonuclear (He2, Ne2, Ar2) and three heteronuclear (He–Ne, He–Ar, Ne–Ar) rare gas dimers are presented. The curves were calculated using several correlation consistent basis sets and the supermolecule single and double excitation coupled-cluster theory with noniterative perturbational treatment of triple excitations, CCSD(T). The most accurate results were obtained with the aug-cc-pV5Z basis set supplemented with an additional (3s3p2d2f1g) set of bond functions. The results obtained with a smaller aug-cc-pVQZ+(3s3p2d2f1g) basis set are almost as accurate. Both basis sets give results in better agreement with potentials based on experiments than the recent results obtained with larger d-aug-cc-pV6Z and t-aug-cc-pV6Z basis sets but without bond functions. For each complex and each basis set a fitted potential energy curve is given. In addition, for each complex, with the exception of He2, the values of Re, De, B0, D0, and 〈R〉0 are given. For He2 no bound states were found ...

An ab initio study of the potential energy surface and spectrum of Ar–CO

The two-dimensional potential energy surfaces for the Ar–CO complex have been developed using single and double excitation coupled-cluster theory with noniterative treatment of triple excitations [CCSD(T)]. The most accurate results have been obtained with the augmented correlation-consistent polarized triple zeta basis set (aug-cc-pVTZ) with an additional (3s3p2d2f1g) set of bond functions. The minimum of −104.68 cm−1 has been found at (R,Θ)=(3.714 A, 92.88°), where R and Θ denote the Jacobi coordinates with Θ=0° corresponding to the linear Ar–OC geometry and Θ=180° to the linear Ar–CO geometry. Dynamical calculations have been performed to determine the frequencies of various rotational and rovibrational transitions. The overall agreement with experiment is good. For example, the calculated frequencies of the intermolecular bending and stretching vibrations, 12.015 and 18.520 cm−1, respectively, agree very well with the experimental values (12.014 and 18.110 cm−1).

Theoretical studies of the X̃ 2Π and à 2Σ+ states of the He⋅OH and Ne⋅OH complexes

The two-dimensional potential energy surfaces for the X 2Π and A 2Σ+ states of the He⋅SH and Ne⋅SH complexes have been calculated using the restricted open-shell coupled cluster theory [RCCSD(T)] and the triple-zeta augmented correlation consistent polarized basis sets with an additional (3s3p2d2f1g) set of bond functions. In the case of the A 2Σ+ state of Ne⋅SH the entire surface has also been developed using the quadruple-zeta basis set with bond functions as exploratory calculations demonstrated significant differences between the RCCSD(T) results obtained with the triple- and quadruple-zeta basis sets. These potentials are somewhat shallower and less anisotropic in comparison to the surfaces for the related He⋅OH and Ne⋅OH complexes. In contrast to He⋅OH and Ne⋅OH, we find that the linear Rg–SH (Rg=He, Ne) configurations are in all but one case lower in energy than the Rg–HS geometries. Variational calculations of the bound rotation-vibration states have been performed using Hamiltonians that include...

Theoretical study of the He–HCN, Ne–HCN, Ar–HCN, and Kr–HCN complexes

The two-dimensional potential energy surfaces for the He–HCN, Ne–HCN, Ar–HCN, and Kr–HCN complexes are presented. Calculations have been performed using single and double excitation coupled-cluster theory with noniterative treatment of triple excitations [CCSD(T)] and the augmented correlation-consistent polarized triple-zeta basis set (aug-cc-pVTZ) with an additional (3s3p2d2f1g) set of bond functions. The potentials have been used to find the vibration–rotation energies of the four complexes and their deuterated analogs. The frequencies of rotational or rovibrational transitions found for He–HCN and Ar–HCN are in very good agreement with the experimental results. Good agreement is also obtained with the experimental rotational transition frequencies for Kr–HCN. For Ne–HCN, on the other hand, the agreement with the experimental data is not as good, but can be improved by using larger basis sets.

An analysis of the electrostatic interaction between nucleic acid bases.

Results from several commonly used approximate methods of evaluating electrostatic interactions have been compared to the rigorous, nonexpanded electrostatic energies at both uncorrelated and correlated levels of theory. We examined a number of energy profiles for both hydrogen bonded and stacked configurations of the nucleic acid base pairs. We found that the penetration effects play an extremely important role and the expanded electrostatic energies are significantly underestimated with respect to the ab initio values. Apart from the inability to reproduce the magnitudes of the ab initio electrostatic energy, there are other problems with the available approximate electrostatic models. For example, the distributed multipole analysis, one of the most advanced methods, is extremely sensitive to the basis set and level of theory used to evaluate the multipole moments. Detailed ab initio results are provided that other researchers could use to test their approximate models.