Recently, a method has been developed which uses experimental crystallographic data to construct orbitals yielding essentially the exact density for the crystals. The wavefunction obtained by this method is a Slater determinant. By using the Colle‐Salvetti method we show the correlation energy can be calculated from this Slater determinant. We apply the method to the case of the beryllium atom. Copyright © 1985 John Wiley & Sons, Inc.