Forces in pseudopotential molecular calculations

The forces derived from the Hellmann-Feynman theory are calculated for a molecule, using the local density approximation and within the pseudopotential scheme. We show that, in contrast to the general result of all-electrons calculations, the pseudopotential scheme allows us to obtain accurate and reliable forces with a small number of basis functions. Fast convergence in the basis set is crucial for calculations of large molecules. We apply our method to the Na₂ molecule and discuss the convergence rate in detail.