RHF open-shell derivatives are normally calculated using Liotard's analytical C.I. method . In general, do not use NOANCI (NO ANalytical Configuration Interaction derivatives). NOANCI should only be used if there is cause to believe that the derivatives are faulty, because it evaluates the gradient using finite differences involving complete SCF calculations. In contrast, Liotard's method is very fast, so using NOANCI will cause the job to take much longer than if NOANCI were not used.
In the unusual situation when analytical C.I. derivatives are not to be used, specify NOANCI. See also GRADIENTS.
NOANCI can also be used for closed-shell RHF calculations when there is a need to avoid the standard diatomic derivatives. Thus, when PM6-DH2 is used, there are known small errors in the gradient calculation that prevent the exact energy minimum being reached. When these errors are avoided by using NOANCI, the exact energy minimum is obtained.