Quantum Monte Carlo study of ground and first excited state of C, N, O, F, and Ne atoms using Slater‐Jastrow‐Backflow wave function.

All‐electron fixed‐node diffusion quantum Monte Carlo energies of the two lowest‐lying states of C, N, O, F, and Ne atoms are reported. The Slater‐Jastrow form is used as the trial wave function. We will use single‐ and multideterminant wave functions as the Slater part. The single‐determinant wave function has been computed by the Hartree‐Fock method and the multideterminant wave functions have been computed by the complete active space self‐consistent field, configuration interaction with single and double excitation, configuration interaction with single, double, triple, and quadruple excitation and second‐order configuration interaction. For the ground‐ and first excited states, the multideterminant wave functions have computed more than 99% of the correlation energy. Significant improvements have been achieved using the backflow transformations and up to 99.8% of the correlation energy has been recovered. A very good agreement with the experimental data has been obtained for the excitation energies. [ABSTRACT FROM AUTHOR]