Two-electron properties for the beryllium atom from explicitly correlated wavefunctions

Two-body properties of the ground state of beryllium atom in position and momentum spaces are obtained starting from a compact and accurate explicitly correlated wavefunction. The wavefunction is factorized into a correlation factor, depending on the interelectronic distance, and a multi configuration expansion. The variational Monte-Carlo method (VMC) has been used to optimize the trial wavefunction and to compute the different properties. The results are compared with others calculated from large configuration interaction expansions, multi configuration self-consistent field wavefunctions and explicitly correlated gaussian expansions, showing good performance of the results of this work. The use of explicitly correlated wavefunction along with the VMC provides reliable results for complex atomic systems as the beryllium atom.