A better multi-reference Davidson correction for internally contracted and uncontracted multi-reference configuration interaction calculations.

We propose a new multi-reference Davidson correction and show that it gives a much better prediction of the role of quadruple excitations compared to existing corrections. However, due to the neglect of triple excitations, our most promising correction is a hybrid scheme whereby the average of the new correction and a conventional correction is used. This hybrid correction performs well in a wide variety of cases, including the avoided crossing of two roots of the same symmetry.