Analytic gradient for the relativistic elimination of small components (RESCs) approach

An analytic gradient technique for the relativistic elimination of small components (RESCs) approach has been successfully derived and implemented. An internal uncontraction scheme has been added to both energy and gradient, thus improving the resolution of the identity quality. The gradient technique is a full analytic derivative of the energy expression, including the resolution of the identity. A comparison to other similar relativistic methods has been performed on the following molecules: W(CO)6,Os(CO)5,Pt(CO)4 and good agreement to other methods is achieved, within the limitations of the basis set and electron correlation level (MP2).