Accurate ab initio potentials at low cost via correlation scaling and extrapolation: application to CO(A 1Pi).

A recently proposed scheme that enables high quality molecular potentials to be obtained from small basis set calculations via scaling and extrapolation of the electron correlation to the complete basis set limit plus extrapolation to the complete basis set limit of the complete-active-space self-consistent field energy has been applied to the A 1Pi electronic state of CO. Based on standard multireference configuration interaction raw energies calculated with correlation-consistent basis sets of the aug-cc-pVXZ and aug-cc-pCVXZ types for the scaling and extrapolation procedures, a barrier separating the equilibrium region from the dissociation asymptote of 594+/-46 cm(-1) has been predicted at an internuclear distance of 2.25+/-0.01 A in reasonable agreement with some previous theoretical work and a recent direct least-squares fit to available spectroscopic data, while being 38% smaller than an early experimental upper limit of 950+/-150 cm(-1). Good agreement with experiment is also obtained for the minimum well depth and location at equilibrium: 25,532+/-165 cm(-1) at 1.2346+/-0.0015 A (theoretical) versus 25,597 cm(-1) at 1.2346+/-0.0006 A (experimental). The method can be extended to other wave function models, providing a general strategy for accurate potentials of larger dimensionality at costs that can be drastically smaller than traditional ones.