  The analytic energy gradients with respect to nuclear motion are derived for
natural orbital functional (NOF) theory. The resulting equations do not require
to resort to linear-response theory, so the computation of NOF energy gradients
is analogous to gradient calculations at the Hartree-Fock level of theory. The
structures of 15 spin-compensated systems, composed by first- and second-row
atoms, are optimized employing the conjugate gradient algorithm. As
functionals, two orbital-pairing approaches were used, namely, the fifth and
sixth Piris NOFs (PNOF5 and PNOF6). For the latter, the obtained equilibrium
geometries are compared with coupled cluster singles and doubles (CCSD)
calculations and accurate empirical data.
