A development version of CPMD 4.3 has been used for all MP2 calculations with PWs , in combination with hard normconserving Goedecker-Teter-Hutter (GTH) pseudopotentials specifically parametrized for HF . The HF wavefunction has been optimized with either DIIS or preconditioned conjugate gradient optimization up to a maximum residual component of the gradient on occupied orbitals lower than 10 -7 a.u., respectively 10 -5 a.u. for the n max virtual orbitals obtained via subsequent Davidson diagonalization .