P.E.S. Wormer

Density functional calculations of molecular hyperfine interactions in the zero order regular approximation for relativistic effects

Expressions are derived within the relativistic regular approximation (ZORA) for the evaluation of the magnetic hyperfine interactions in paramagnetic molecules. For hydrogen-like atoms exact first order relations between the ZORA and Dirac formalism are given for the calculation of g- and A-tensors. Density functional calculations are performed on the neutral atoms Cu, Ag and Au, on some small test molecules NO2, HCO, and TiF3, and on some paramagnetic clusters consisting of 5 or 7 atoms of the group IB metals: Cu7, Cu2Ag5, CuAg6, Ag5, Ag7, and Au7. It is shown that the calculated ESR parameters of the heptamers are in good agreement with results of experiments, which originally were assigned to pentamers.

Density functional calculations of molecular g-tensors in the zero order regular approximation for relativistic effects

A method has been developed for the calculation of the g-tensor of Kramers doublet open shell molecules, which uses the spinor of the unpaired electron of the paramagnetic molecule, obtained from a density functional calculation. Spin–orbit coupling is taken into account variationally using the zeroth-order regular approximation (ZORA) to the Dirac equation. The problem of gauge dependence is solved by using gauge including atomic orbitals (GIAO’s). The method gives fair agreement with experimental values for the g values of some small test molecules NO2, HCO, and TiF3.

Ab initio studies of the interactions in Van der Waals molecules

2 Mechanisms of Van der Waals Interactions; Distance and Orientational Dependence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.1 Distance and orientational dependence 4 2.2 Model potentials 6 2.3 Contributions to the interaction energy 8 2.3.1 Electrostat ic; long range mult ipole interactions, penetrat ion effects 8 2.3.2 Induct ion, dispersion; muit ipole interactions, penetra t ion effects 10 2.3.3 Exchange . . . . . . . . . . . . . . . . . . . . . . . . . 12 2.4 Interact ions from supermolecule calculations . . . . . . . . . . . . 13 2.5 Addi t iv i ty . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Time‐dependent coupled Hartree–Fock calculations of multipole polarizabilities and dispersion interactions in van der Waals dimers consisting of He, H2, Ne, and N2

The long‐range dispersion interaction coefficients for van der Waals dimers consisting of He, Ne, H2, and N2 have been computed in the time‐dependent coupled Hartree–Fock approximation. Static multipole polarizabilities and van der Waals coefficients Cn(n=6,8,10) are presented. The difference between coupled and uncoupled Hartree–Fock results (‘‘apparent’’ correlation) is large in all systems considered and only in the case of Ne the ‘‘true’’ correlation effects are larger. In order to keep the basis set errors in the computed properties smaller than the correlation errors, the basis sets have to be very large. This is demonstrated by using different basis sets for the molecules H2 and N2. The computed van der Waals coefficients for the ten dimers are very accurate, at least the C6 and C8 coefficients, with correlation errors less than 6% for He2, H2–H2, N2–N2, HeH2, HeN2, and H2–N2.

Water pair potential of near spectroscopic accuracy. I. Analysis of potential surface and virial coefficients

A new ab initio pair potential for water was generated by fitting 2510 interaction energies computed by the use of symmetry-adapted perturbation theory (SAPT). The new site–site functional form, named SAPT-5s, is simple enough to be applied in molecular simulations of condensed phases and at the same time reproduces the computed points with accuracy exceeding that of the elaborate SAPT-pp functional form used earlier [J. Chem. Phys. 107, 4207 (1997)]. SAPT-5s has been shown to quantitatively predict the water dimer spectra, see the following paper (paper II). It also gives the second virial coefficient in excellent agreement with experiment. Features of the water dimer potential energy surface have been analyzed using SAPT-5s. Average values of powers of the intermolecular separation—obtained from the ground-state rovibrational wave function computed in the SAPT-5s potential—have been combined with measured values to obtain a new empirical estimate of the equilibrium O–O separation equal to 5.50±0.01 bohr...

Ab initio dispersion coefficients for interactions involving rare‐gas atoms

Calculations of the dynamic dipole, quadrupole, and octopole polarizabilities of Ne, Ar, Kr, and Xe are carried out using both time‐dependent coupled Hartree–Fock and many‐body perturbation theory methods. Dispersion coefficients are calculated for interactions involving these species. The dynamic polarizabilities are combined with previously published dynamic polarizabilities of H, He, H2, N2, HF, and CO to obtain dispersion coefficients for the interactions involving one of these species and one of Ne, Ar, Kr, or Xe. The dipole–dipole dispersion coefficients agree quite well with the best available semiempirical estimates. The isotropic higher multipole coefficients are in reasonable agreement with previous semiempirical estimates where available, and the anisotropic ones are, in most cases, the first reliable ones to appear in the literature. Nonadditive three‐body dispersion coefficients for the Ne3, Ar3, Kr3, and Xe3 interactions are also calculated.

An ab initio intermolecular potential for the carbon monoxide dimer (CO)2

We have constructed an analytical potential energy surface for CO–CO by means of ab initio calculations for the electrostatic and first‐order exchange interactions and by the use of accurate dispersion coefficients recently calculated in our group. Parameter‐free damping functions account for second‐order exchange and penetration effects. The anisotropy of this potential is represented by an expansion in spherical harmonics for the molecules A and B, up to LA, LB=5 inclusive. The second virial coefficients calculated with this potential, including quantum corrections, lie within the experimental error bars over a wide temperature range.

Many-body perturbation theory of frequency-dependent polarizabilities and van der Waals coefficients : application to H2O-H2O and Ar-NH3

Correlation contributions to the multipole moments and frequency dependent polarizabilities of molecules are described within the framework of time‐dependent coupled Hartree–Fock and many‐body perturbation theory. Computationally feasible expressions are given for the ‘‘true’’ correlation contributions to the multipole moments and frequency dependent polarizabilities. The polarizabilities of argon, ammonia and water and the van der Waals induction and dispersion coefficients of H2O–H2O and Ar–NH3 are presented.

A new He–CO interaction energy surface with vibrational coordinate dependence. I. Ab initio potential and infrared spectrum

The intermolecular potential energy surface of the He–CO complex including the CO bond length dependence has been calculated using symmetry-adapted perturbation theory (SAPT). The potential has a minimum of em=−23.734 cm−1 with Rm=6.53 bohr at a skew geometry (ϑm=48.4°) if the molecular bond length is fixed at the equilibrium value of 2.132 bohr. We have applied the potential in the calculation of bound state levels and the infrared spectrum for the 3He–CO and 4He–CO complexes. The computed ab initio transition frequencies are found to agree within 0.1 cm−1 with experiment. In paper II [J. P. Reid, H. M. Quiney, and C. J. S. M. Simpson, J. Chem. Phys. 107, 9929 (1997)], the potential surface is used to calculate vibrational relaxation cross sections and rate constants.

Correlated van der Waals coefficients for dimers consisting of He, Ne, H2, and N2

Time‐dependent coupled Hartree–Fock frequency‐dependent polarizabilities have been corrected for true correlation effects by means of many‐body perturbation theory. Polarizabilities have been computed for the monomers He, Ne, H2, and N2 through second order in the correlation potential. With these polarizabilities as input the van der Waals coefficients of all possible dimers have been obtained by the use of the Casimir–Polder relation.