Marina Rui

Long-range dispersion coefficients for like centrosymmetric linear molecules and an application to H2H2

Abstract The real spherical tensor theory of long-range intermolecular coefficients developed in previous papers is applied to derive explicit formulae for the first three dispersion coefficients for like centrosymmetric linear molecules. The expansion of angledependent coefficients in associated Legendre polynomials allows one to identify the isotropic and anisotropic components of the dispersion interaction in terms of London dispersion constants, the treatment of higher coefficients being simplified by the coupling of the elementary (1, 1′)-polarizations to resultant angular momenta LA and LB onto each molecule. The contributions from all coupling schemes are given explicitly for C6, C8, c10, and numerical results are presented for H2H2 using two-term reduced spectra values from the Kaiserlautern group.

Bond-bond pair calculations of rotational barriers

Ab initio STO-3G calculations of rotation barriers in six single-rotor molecules have shown the inadequacy of the Mulliken approximation in the Surjan formula which expresses barriers as a sum of bond-bond pair interactions. Following our early work, expansion to order S2 followed by pair delocalization is suggested as a possible alternative to semi-empirical advance.

Bond-orbital study of the angular geometry of small polyatomic molecules

Abstract Ab initio STO-3G bond-orbital (BO) studies of the angular geometry in CH 4 , NH 3 , H 2 O, CH 2 O, CH 3 F, CH 2 F 2 show that the small second-order delocalization between non-orthogonal SCF BOs is essential to give reasonably accurate results when lone pairs are present on the central atom.

Long-range induction coefficients for like centrosymmetric linear molecules and an application to H2–H2

Abstract The C 8 and C 10 long-range induction coefficients for two like centrosymmetric linear molecules are derived explicitly using the irreducible properties of spherical tensor operators. An application to the H 2 –H 2 system shows that whereas the isotropic induction coefficients are 50–100 times smaller than dispersion, the induction anisotropies γ n are always larger than the corresponding dispersion anisotropies.

One-centre multipole calculation of second-order perturbation energies for H+ 2

A knowledge of the H(1s) pseudospectrum allows one to calculate the one-centre second-order perturbation energies for the H(1s)-H+ interaction. The efficiency of such calculations is tested in detail for induction, exchange-induction, first-order overlap and Murrell-Shaw-Musher-Amos second-order energies for the ground and excited state H+ 2 at different internuclear distances. It is found that a large number of STO functions accounting for at least the first sixteen non-expanded multipoles is needed to obtain three figure accuracy in the short to medium range of internuclear separations. Higher accuracy can be obtained only in terms of a sensibly smaller number of two-centre functions which automatically satisfy the electron-nucleus cusp requirement.

On the feasibility of bond pair calculations of rotational barriers: a preliminary investigation

Abstract The feasibility of the previously proposed bond pair approximation is examined for rotational barriers upon expansion of the penetration energy to order S 2 . Ab initio non-orthogonal bond orbital calculations on five single rotor XH m YH n molecules show that many-body purely electrostatic effects are unimportant but three-body non-additivity due to overlap is non-negligible for quantitative predictions.

LONG-RANGE DISPERSION AND INDUCTION COEFFICIENTS FOR THE HOMODIMERS OF LI2, NA2 AND K2

Abstract Long-range dispersion and induction coefficients for the homodimers of Li2, Na2, K2 are evaluated in the LALBM scheme using explicit formulae presented previously and numerical values of moments, polarizabilities and dispersion constants obtained from 2-term reduced spectra of the Kaiserslautern group. While induction coefficients are 5–8 times smaller than dispersion, induction anisotropies are always larger than the corresponding dispersion anisotropies for all homodimers.

PARTITIONED ORBITALS FOR THE PERTURBATIVE EVALUATION OF GROUND STATE HYDROGEN ATOM INDUCTION ENERGIES

A new method has been developed for the evaluation of damped multipole contributions to the second order perturbative energies induced by molecular potentials on the ground state of hydrogen atoms. It involves non-conventional discontinuous atomic orbitals partitioned in their radial dependence, giving rise, however, to continuous first-order wavefunctions with continuous first derivatives. Small bases consisting of partitioned orbitals have been found sufficient to yield nearly exact evaluations of the first multipole contributions to the secondorder H–H+ induction energy, over a wide range of internuclear distances.