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A new 155-term electronic wave function for the ground state of the hydrogen molecule has been selected which gives a more accurate Born–Oppenheimer potential energy curve in the range 3.2≤R≤7.2 bohr. The largest improvement 0.112 cm-1 has been found for R=4.8 bohr. The adiabatic corrections have also been calculated in the full range of R giving more accurate values. These new results have been employed to calculate the dissociation energies and ionization potentials for H2, HD, and D2. The agreement between resulting dissociation energies and recently published experimental data are excellent; the discrepancy is within the experimental error limits. For the ionization potential the agreement is fairly good, although not ideal. There is still room for improvement from both theoretical and experimental side. [ABSTRACT FROM AUTHOR]

Presents a symplectic method for rigid-body molecular dynamics simulation. Evolution of the entire rotation matrix; Use of a fast approximate Newton solver to preserve the orthogonality of the rotation matrix; Testing of the method on a system of soft-sphere dipoles; Comparison with quaternion evolution.

The Born–Oppenheimer potential energy curve for the electronic ground state of the hydrogen molecule has been computed for internuclear distances 0.2≤R≤12.0 bohr. At all R the energies are lower than any previously reported values. At the equilibrium distance the improvement is negligible; its largest value, 0.47 cm-1, has been obtained for R=2.4 bohr. Vibrational energies have been computed for all isotopic species using the new potential energy curve. For H2, HD, and D2 the recent nonadiabatic corrections of Wolniewicz have been included. The resulting dissociation energies of H2 and HD agree with the experimental values within the experimental error limits; for D2 the discrepancy amounts to 0.6±0.3 cm-1. [ABSTRACT FROM AUTHOR]

The generalized Heitler–London perturbation theory for molecular interaction energies proposed recently by Tang and Toennies [J. Chem. Phys. 95, 5981 (1991)] is proved to be equivalent to the symmetrized Rayleigh–Schrödinger perturbation expansion. This theory is applied to the interaction of two hydrogen atoms and is shown to reproduce the interaction energy in the region of the van der Waals minimum to within about 1% already in the third-order treatment. It is shown that the convergence radius of this theory is the same as that of the polarization perturbation theory, i.e., is marginally greater than unity at large interatomic distances R. This proximity to unity results in a prohibitively slow high-order convergence of the expansion in the region of the van der Waals minimum. Consequently, for the singlet state at large R a small part of the exchange energy cannot be recovered in practice by a direct term-by-term summation of the series. The perturbation series resulting from the application of the theory to the antisymmetric triplet state of H2 converges to an unphysical energy lying above the energy of the triplet state. At large R the difference between the physical energy and the unphysical limit of the series is very small and can be neglected in practical applications of the theory. [ABSTRACT FROM AUTHOR]

We have performed a study of the water dimer interaction using larger basis sets and higher levels of theory than have been previously applied to this system. For the minimum geometry we have used spdf basis sets containing up to 212 orbitals. Our most accurate SCF interaction energy for the minimum is -3.73±0.05 kcal/mol. We have shown that this energy can be reproduced to within 0.1 kcal/mol using much smaller basis sets containing proper (diffuse) exponents. Accounting for the basis set superposition error is shown to be essential. We computed the dispersion energy with neglect of the intramolecular correlation using basis sets of various sizes. The best value obtained in a large spdf basis set with exponents which optimize this quantity is -1.93 kcal/mol and it is expected to be accurate to 0.1 kcal/mol or better. Using some of these basis sets we have performed supermolecular many-body perturbation theory (MBPT) and coupled-cluster (CC) calculations including triple excitations. We have shown that if the correlated supermolecular interaction energies are not corrected for the basis set superposition error, the correlated part of the interaction energy varies widely with the basis set. In contrast, the corrected values converge smoothly. On the basis of the dispersion energy and the MBPT/CC results we predict the correlated part of the interaction energy to be -1.0±0.3 kcal/mol, which leads to the total interaction energy of -4.7±0.35 kcal/mol. Thus, our work favors the lower limit of the experimentally predicted interaction energy of -5.4±0.7 kcal/mol. [ABSTRACT FROM AUTHOR]

Ground state energy of the hydrogen molecule, at the equilibrium internuclear separation, has been computed in the Born–Oppenheimer approximation. The generalized James–Coolidge expansion has been used for the variational wave function. The expansion length has been systematically extended, and from the results a limiting value of the energy has been obtained which is by 0.009 cm-1 lower than the best variational value. Improved agreement is obtained with experimental values of the ionization potential for H2 and HD. For D2 the disagreement between theoretical and experimental values of the ionization potential seems to be significant. [ABSTRACT FROM AUTHOR]