Tools for analysing configuration interaction wavefunctions
P. Delaneya, J. C. Greer,
aNMRC, Prospect Row, Cork, Ireland.
P. Delaneya, J. C. Greer, , a
The configuration interaction (CI) approach to quantum chemical calculations is a well-established means of calculating accurately the solution to the Schr"odinger equation for many-electron systems. It represents the many-body electron wavefunction as a sum of spin-projected Slater determinants of orthogonal one-body spin-orbitals. The CI wavefunction becomes the exact solution of the Schrödinger equation as the length of the expansion becomes infinite, however, it is a difficult quantity to visualise and analyse for many-electron problems. We describe a method for efficiently calculating the spin-averaged one- and two-body reduced density matrices rho_Psi ( r; r' ) and Gamma_Psi ( r_1, r_2 ; r'_1, r'_2 )$ of an arbitrary CI wavefunction Psi. These low-dimensional functions are helpful tools for analysing many-body wavefunctions; we illustrate this for the case of the electron-electron cusp. From rho and Gamma one can calculate the matrix elements of any one- or two-body spin-free operator O. For example, if O is an applied electric field, this field can be included into the CI Hamiltonian and polarisation or gating effects may be studied for finite electron systems.