DALTON do not have any support for the calculation of molecular gradients and Hessians for the non-variational wave functions MP2 and CI . However, in order to exploit the facilities of the first-order geometry optimization routines in DALTON , a numerical gradient based on energies will be calculated if a geometry optimization is invoked for a non-variational wave function. Thus, to optimize the MP2 geometry of a molecule, the only input needed is
**DALTON INPUT .MINIMIZE *MINIMIZE .1STORDER **WAVE FUNCTION .HF .MP2 *END OF INPUT
The size of the displacements used during the evaluation of the
numerical gradient can be controled through the keyword .DISPLA
in the *MINIMI
input module. Default value is 
a.u. By default, the threshold for convergence of the geometry will be
changed because of estimated inaccuracies in the numerical
gradients. However, if the threshold for convergence is altered
manually, this user supplied threshold for convergence will be used
also in geometry optimizations using numerical
gradients . Note that
due to the possibility of larger numerical errors in the gradient, too
tight convergence criteria for an optimized geometry may make it
difficult for the program to obtained a converged geometry.