Selected configuration interaction method using sampled first-order corrections to wave functions

A new selected configuration interaction (CI) method was proposed for the potential energy surfaces of quasi-degenerate and excited states. Slater determinants are generated by sampling the first-order corrections to the target-state wave functions using the quantum Monte Carlo method in determinant space. As in the Monte Carlo (MC) CI method, the wave function is improved at each iteration by generating new determinants and applying a pruning step. Compared to the random generation in the MCCI calculations, the number of iterations before convergence is significantly reduced. Regarding the potential energy curves of the ground and excited states of C2, the non-parallelity errors were sufficiently small, thus indicating the method’s applicability to the calculations of potential energy surfaces