Analytical gradients Anharmonicity

Incorporation of electron correlation in the determination of anharmonic force fields is necessary if ab initio predictions are to be made reliable under general circumstances. Since determination of even complete quartic force fields for larger molecules is not feasible without analytic derivative methods, this places a severe limitation on the size of molecular systems which can be studied. While analytic gradient techniques (see Gradient Theory) have been implemented for most state-of-the-art correlation methods, efficient analytic second-derivative... 

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