The implementation of any self-consistent exchange-correlation functional can be checked by comparing numerical and analytic forces arising from microscopic displacements of nuclei within an arbitrary system. The agreement between the two forces demonstrates that the XC energy functional is consistent with the potential.
The analytic forces are obtained from a theorem due to Hellmann  and Feynman , which states that when the valence electron wavefunctions are variationally optimised, the physical force on an ion is simply the classical electrostatic force due to the electrons and nuclei. Due to the extensive basis sets used in plane-wave calculations, the energy is essentially variational and so the force should be given by the same Hellmann-Feynman expression. In other basis set calculations, such as those using localised Gaussian functions, the basis size is smaller and so the Hellmann-Feynman theorem does not hold. An additional term known as the Pulay force  must be calculated in these cases.
The HCTH implementation was checked by calculating forces within the
molecule, using a bond length of Å. The numerical
in the molecule was determined from
the displacement of one of the ions through a distance
from its equilibrium position ,
Using a well converged cutoff energy, the numerical and analytic Hellmann-Feynman forces agreed to within , for a displacement of Å. The fact that this is a very small discrepancy and that the same degree of accuracy was also obtained with the previously established PW91 functional in CASTEP, demonstrates the correct implementation of the HCTH functional. The spin-polarised version of HCTH was also checked using the open shell molecule.