Approaching chemical accuracy using full configuration-interaction quantum Monte Carlo: A study of ionization potentials

摘要

A new quantum Monte Carlo u0001QMCu0002 method is used to calculate exact, fullnconfiguration-interaction u0001FCIu0002 energies of the neutral and cationic elements from Li to Mg, in anfamily of commonly used basis sets. Annihilation processes between positive and negative walkersnenable the exact N-electron wave function to emerge as a linear superposition of the u0001factoriallynlargeu0002 space of Slater determinants, with individual determinants being stochastically sampled. As anresult, extremely large spaces u0001exceeding 1015 determinantsu0002 become accessible for FCIncalculations. No fixed-node approximation is necessary, and the only remaining source of error isnthe one-electron basis set, which can be systematically reduced by enlargement of the basis set. Wenhave investigated the family of aug-cc-pVXZ Dunning basis sets up to X=5. The resultingnionization potentials are—with one exception u0001Nau0002—consistently accurate to within chemicalnaccuracy. The anomalous case of Na suggests that its basis set may be improvable. Extrapolationnschemes are examined as a way of further improving the values obtained, and although annimprovement is seen in the mean-absolute error, the results of extrapolation are not uniformly betternthan the largest basis set calculations reported. More generally, these results demonstrate the utilitynof the QMC method to provide FCI energies for realistic systems and basis sets.