We have delineated certain important aspects of the use of theoretical Compton profiles for estimating cohesive energy, energy of mixing etc. Inclusion of electron-electron correlation in the calculation of Compton profiles seems to be essential in order to get the correct values of ground state energy differences. We have also done some model calculation for the energy change as a result of 'switching on' correlation in a homogeneous electron gas at zero temperature. Numerical estimates of this 'correlation energy' have been obtained for free-electron-like metals, viz Na and Li.