Energetics of carbon clusters C-8 and C-10 from all-electron quantum MonteCarlo calculations

The energetics of carbon clusters C-8 and C-10 in several configurations ha ve been determined in all-electron fixed-node quantum Monte Carlo calculati ons. The total electronic energies obtained are 0.4-1.2 hartrees lower than those of the lowest-energy analytic variational and coupled-cluster calcul ations, and they are only 0.2 to 0.4 hartrees above the experimentally base d total energies. The recovery of correlation energy is 89%-94%. For C-8, r elative values are found to be in agreement with earlier calculations: the cyclic C-4h (1)A(g) structure, the linear (3)Sigma(g)(-) (cumulenic), and t he linear (1)Sigma(g)(+) (cumulenic) structures are found lowest in energy and within 10 kcal/mol of each other. For C-10, relative values are found t o be in agreement with earlier calculations: the cyclic D-5h (1)A(1)' (dist orted cumulenic) and D-10h (1)A(1g) (cumulenic) structures are found to be lowest in energy, within 5 kcal/mol of each other, 60 kcal/mol below the li near (3)Sigma(g)(-) (cumulenic) structure. An analysis of the scaling of co mputational effort with the number of electrons N-e for these quantum Monte Carlo calculations indicates a third-power dependence N-e(3). (C) 1999 Ame rican Institute of Physics. [S0021-9606(99)30122-7].