Quantum Monte Carlo study of the classical barrier height for the H + H/sub 2/ exchange reaction: restricted versus unrestricted trial functions
The fixed-node quantum Monte Carlo (QMC) method is used to obtain the classical barrier height for the H + H/sub 2/ exchange reaction. Using a spin-restricted, single-determinant trial function Psi/sub T/, it was found that the reaction barrier E/sub b/ is less than 9.69 +/- 0.25 kcal/mol. This mean value is below the calculated energy barrier obtained by Liu in the most extensive configuration interaction (CI) calculations on this system. Furthermore, the QMC saddle-point energy of -1.65903 +/- 0.00040 hartrees at the CI-determined geometry lies 0.00027 a.u. (0.17 kcal/mol) below Liu's best CI value. Finally, spin-restricted and spin-unrestricted single-determinant trial functions are contrasted. Although the variational energy (Psi/sub T/absolute value H Psi/sub T/) for an SCF Psi/sub T/ must be lower for the unrestricted case, this is not true generally for QMC. In fact, the authors show 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/. 14 references.
- Research Organization:
- Lawrence Berkeley Lab., CA
- DOE Contract Number:
- AC03-76SF00098
- OSTI ID:
- 5497387
- Journal Information:
- Int. J. Quant. Chem., Symp.; (United States), Vol. 18
- Country of Publication:
- United States
- Language:
- English
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