Variationally determined electronic states for the theoretical analysis of intramolecular interaction: I. Resonance energy and rotational barrier of the C-N bond in formamide and its analogs

We present a new method that produces a variationally determined zeroth-order wave function for the analysis of intramolecular interactions between the fragments of a molecule. In our method, called the spacerestricted wave function (SRW) method, this wave function is defined with nonorthogonal orbitals, which are obtained using the appropriately restricted variational spaces. The wave function thus obtained represents the electronic state with the target interactions deactivated, and it is constructed without unnecessarily breaking bonds, in contrast to some of the existing methods. Furthermore, we can perform energy decomposition analysis of intramolecular interactions using the zeroth-order functions that the SRW method yields. The validity of the SRW method is demonstrated in the analysis of the resonance energy and the rotational barrier of the C-N bond in formamide and its analogs. This method gives energy components that are different from those given by existing methods. With these components, we elucidate the origin of the rotational barrier from another point of view. Our SRW method gives meaningful results for the investigation of electron behavior and the nature of the molecular system.