Showing papers by "Poul Jo published in 1994"

Calculation of size‐intensive transition moments from the coupled cluster singles and doubles linear response function

Abstract: Coupled cluster singles and doubles linear response (CCLR) calculations have been carried out for excitation energies and dipole transition strengths for the lowest excitations in LiH, CH+, and C4 and the results compared with the results from a CI‐like approach to equation of motion coupled cluster (EOMCC). The transition strengths are similar in the two approaches for single molecule calculations on small systems. However, the CCLR approach gives size‐intensive dipole transition strengths, while the EOMCC formalism does not. Thus, EOMCC calculations can give unphysically dipole transition strengths, e.g., in EOMCC calculations on a sequence of noninteracting LiH systems we obtained a negative dipole strength for the lowest totally symmetric dipole allowed transition for 19 or more noninteracting LiH systems. The CCLR approach is shown to be a very attractive ‘‘black box’’ approach for the calculation of transition moments.

Multiconfigurational self-consistent field calculations of nuclear shieldings using London atomic orbitals

Abstract: Nuclear shielding calculations are presented for multiconfigurational self‐consistent field wave functions using London atomic orbitals (gauge invariant atomic orbitals). Calculations of nuclear shieldings for eight molecules (H2O, H2S, CH4, N2, CO, HF, F2, and SO2) are presented and compared to corresponding individual gauges for localized orbitals (IGLO) results. The London results show better basis set convergence than IGLO, especially for heavier atoms. It is shown that the choice of active space is crucial for determination of accurate nuclear shielding constants.

Solvent induced polarizabilities and hyperpolarizabilities of para‐nitroaniline studied by reaction field linear response theory

Abstract: Polarizabilities and hyperpolarizabilities and their frequency dispersions are calculated for para‐nitroaniline immersed in solution using a reaction field response model. Single‐ and multiconfigurational wave functions are self‐consistently optimized in the presence of the reaction field of the solvent and used as reference states for response calculations of the dipole spectra, polarizabilities, and hyperpolarizabilities at zero and finite frequencies. The hyperpolarizabilities are obtained either by finite field generated analytical first polarizability calculations or using a two‐state model where all quantities are calculated from the reaction field response method. The effect of the polarity of the solvent on the quantities entering the two‐state model (the ground and charge transfer state dipole and transition moments and transition energy) are investigated in some detail. All are found to contribute to the steep dependence of the hyperpolarizability as a function of the polarity of the solvent. The relative variation of the hyperpolarizability with the polarity of the solvent is well reproduced at correlated level calculations.

Frequency‐dependent polarizabilities of O2 and van der Waals coefficients of dimers containing O2

Abstract: We report frequency‐dependent dipole and quadrupole polarizabilities of the oxygen molecule in its 3Σ−g ground state. These properties are obtained by means of the multiconfiguration self‐consistent field method and used for the computation of the van der Waals coefficients of (O2)2 and the O2–Rg dimers, where Rg is He, Ne, Ar, and Kr. The required frequency‐dependent polarizabilities of the rare gases were computed earlier by means of second‐order many body perturbation theory.

Basis set convergence of atomic axial tensors obtained from self-consistent field calculations using London atomic orbitals

Abstract: Hartree–Fock calculations of atomic axial tensors (AATs) and atomic polar tensors (APTs) are presented for HF, H2O, NH3, CH4, cyclopropane, carbodiimide, allene, and oxirane. London atomic orbitals are used for the axial tensors. The basis set convergence of the London AATs is found to be considerably faster than for the distributed origins (DO) approach. Indeed, the convergence of the London axial tensors is comparable to that of the polar tensors, indicating that the differential intensities of vibrational circular dichroism can be calculated as accurately as the total intensities of infrared spectroscopy. Dipole and rotational strengths are reported for NHDT, trans‐cyclopropane‐1,2‐d2, carbodiimide, allene‐1,3‐d2, and trans‐oxirane‐2,3‐d2.

Multiconfigurational self‐consistent field calculations of nuclear magnetic resonance indirect spin–spin coupling constants

Abstract: Ab initio calculations of all the nuclear magnetic resonance (NMR) indirect nuclear spin–spin coupling constants in the HN3 molecule and in four isomers of CH2N2 are described. For each molecule, SCF and two multiconfiguration self‐consistent field (MCSCF) wave functions that take into account valence shell correlation effects are applied. All mechanisms contributing to the coupling constants are included. While the SCF results are meaningless, the agreement of the correlated values with those known from experiment is satisfactory. Our most accurate calculations are carried out with the wave functions previously used successfully to compute the NMR shielding constants. All parameters determining the NMR spectra of these molecules have thus been obtained at uniform accuracy from the same reference wave function.