The similarity transformation, one step at a time

My new paper with Matthias Hanuer, Andreas Köhn, and Jürgen Gauss has just appeared in JCP.  The significant contribution of this work is to introduce a new wave function for the internally contracted MRCC method based on a sequential transformation of the reference function (sqic-MRCC).  The sqic-MRCC wave function has the form:

,

where is a multiconfigurational wave function.

Sequencing the exponential operator leads to an advantageous similarity-transformed Hamiltonian, which in turn leads to equations that are less complex with respect to the ic-MRCC method.  What’s most amazing is the fact that the ic-MRCC and sqic-MRCC methods yield almost identical energies.