Quantum Monte Carlo Study of the Classical Barrier Height for the H + H sub 2 Exchange Reaction: Restricted VERSUS Unrestricted Trial Functions

摘要

The fixed-node quantum Monte Carlo (QMC) method is used to obtain the classical barrier height for the H + H sub 2 exchange reaction. With a spin-restricted, single determinant trial function psi/sub T/ the reaction barrier E/sub b/ is less than 9.69 +- 0.25 kcal/mole. This mean value is below the calculated energy barrier obtained by Liu in the most extensive CI calculations on this system. Furthermore, the QMC saddle point energy of -1.6590 +- 0.00040 hartrees at the CI-determined geometry lies 0.00027 a.u. (0.17 kcal/mole) below Liu's best CI value. Finally, spin-restricted and spin-unrestricted single determinant trial functions are contrasted. Although the variational energy for an SCF psi/sub T/ must be lower for the unrestricted case, this is not true generally for QMC. In fact, it is shown that if the unrestricted SCF psi/sub T/ has the lower QMC energy, then there exists another spin-restricted, single-determinant psi/sub T/ whose QMC energy is lower than the QMC energy of the spin-restricted SCF psi/sub T/. (ERA citation 09:031954)