QUANTUM CHEMISTRY BY RANDOM-WALK - H-4 SQUARE

The random‐walk method of solving the Schrödinger equation is applied to the 1B1g and 1B2g states of the H4 square with side length 2.4 a.u. The results provide an independent check of the accuracy of prior variational calculations for these states. With node structures for the 1B1g state taken from a variational calculation with a single‐zeta basis set and taken in the simplest form meeting symmetry requirements, the calculated energies are lower than the expectation value of the energy for the single‐zeta basis set but not as low as the expectation value for an optimized‐exponent double‐zeta basis set with polarization terms. Comparisons of results give no suggestion of a barrier height lower than ∼120 kcal/mol for passage through the square configuration in the exchange reaction H2 + D2 → 2HD. For the 1B2g state with node structure in the simplest form meeting symmetry requirements the calculated energy is ∼65 kcal/mol lower than the expectation value of the energy for variational calculations with a double‐zeta basis set. Copyright © 1979 John Wiley & Sons, Inc.