Theory of Molecular Interaction and Its Application to Studies of Molecular Clusters

用从头算(第一原理)量子化学计算分子相互作用能，必须消除或避免基组展开所固有的误差。产生误差的原因是孤立系统和组合系统的近似不一致。在基于分子轨道(MO)的理论和大多数密度泛函理论(DFT)中，单电子函数(轨道)都是用高斯函数展开的。因此，造成不一致性的原因之一与所使用的轨道有关，这可以被称为轨道基不一致性(OBI)。在分子轨道理论中，多电子波函数用电子组态的叠加或斯莱特行列式的线性组合来描述。因此，不一致的第二个原因是配置基础不一致(CBI)。利用局部投影分子轨道(LPMO)，建立了三阶单激发微扰理论(3rdSPT)和二阶双激发微扰理论(2ndDPT)。该理论通过确定局域条件下的MO来避免OBI，通过限制微扰理论中的电子组态来避免CBI。使用不同的基组对许多分子团簇进行了测试，并将计算的结合能与已发表的参考能进行了比较。所开发的程序可以有效地研究具有3rdSPT/aug-cc-pVDZ的(H2O)25和具有3rdSPT+2ndDPT/Aug-cc-pVTZ的(H2O)6这样的大分子水团簇。

To evaluate the molecular interaction energy by the ab initio (first principle) quantum chemical calculations, the error inherent to the basis set expansions has to be removed or to be avoided. The causes of the error are the inconsistency in the approximation for the isolated systems and for the composite system. In the molecular orbital (MO) based theory and in most of the density functional theory (DFT), the one-electron functions (orbitals) are expanded in terms of Gaussian functions. So one of causes in the inconsistency is related with the orbitals used; which can be called the Orbital Basis Inconsistency (OBI). In MO theory, the many-electron wave functions are described by a superposition of electron configurations, or by a linear combination of Slater determinants. Thus, the second cause of the inconsistency is Configuration Basis Inconsistency (CBI). Using the locally projected molecular orbital (LPMO), the 3rd order single excitation perturbation (3rdSPT) and the 2nd order double excitation perturbation theory (2ndDPT) are developed. The theory can avoid the OBI by determining the MO under the local condition and the CBI by restricting the electron configurations in the perturbation theory. The method is tested for many molecular clusters using various basis sets, and the calculated binding energies are compared with the published reference energies. The developed code is sufficiently efficient to study a large water clusters such as (H_2O)_25 with 3rdSPT/aug-cc-pVDZ and (H_2O)_6 with 3rdSPT+2ndDPT/aug-cc-pVTZ.