Felker Peter M, Bačić Zlatko
Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095-1569, USA.
Department of Chemistry, New York University, New York, New York 10003, USA.
J Chem Phys. 2019 Jul 14;151(2):024305. doi: 10.1063/1.5111131.
We present a method for the efficient calculation of intramolecular vibrational frequencies, and their tunneling splittings, in weakly bound molecular dimers, together with the intermolecular vibrational states within each intramolecular vibrational manifold. The approach involves the partitioning of the dimer's vibrational Hamiltonian into two reduced-dimension Hamiltonians, a rigid-monomer one for the intermolecular vibrations and the other for all intramolecular vibrational degrees of freedom, and a remainder. The eigenstates of the two reduced-dimension Hamiltonians are used to build up a product contracted basis for the diagonalization of the full vibrational Hamiltonian. The key idea is that because of weak coupling between inter- and intra-molecular vibrational modes, the full-dimensional eigenstates in the low-energy portions of the manifolds associated with the intramolecular vibrational excitations can be computed accurately in a compact basis that includes a relatively small number of rigid-monomer intermolecular eigenstates, spanning a range of energies much below those of the intramolecular vibrational states of interest. In the application to the six-dimensional (6D) problem of (HF), we show that this approach produces results in excellent agreement with those in the literature, with a fraction of the basis states required by other methods. In fact, accurate energies of the intramolecular vibrational fundamentals and overtones are obtained using 6D bases that include 4D rigid-monomer intermolecular vibrational eigenstates extending to only 500-1000 cm, far below the HF-stretch fundamental of about 4000 cm. The method thus holds particular promise with respect to calculations on complexes with greater numbers of vibrational degrees of freedom.
我们提出了一种方法，用于高效计算弱束缚分子二聚体中的分子内振动频率及其隧穿分裂，以及每个分子内振动流形中的分子间振动态。该方法包括将二聚体的振动哈密顿量划分为两个降维哈密顿量，一个用于分子间振动的刚性单体哈密顿量，另一个用于所有分子内振动自由度，以及一个余项。利用这两个降维哈密顿量的本征态构建一个乘积收缩基，用于全振动哈密顿量的对角化。关键思想是，由于分子间和分子内振动模式之间的弱耦合，与分子内振动激发相关的流形低能量部分的全维本征态可以在一个紧凑基中精确计算，该基包括相对较少数量的刚性单体分子间本征态，其能量范围远低于感兴趣的分子内振动状态的能量。在应用于（HF）的六维（6D）问题时，我们表明该方法产生的结果与文献中的结果非常吻合，所需的基态数量仅为其他方法的一小部分。事实上，使用包含4D刚性单体分子间振动态的6D基就能获得分子内振动基频和泛频的精确能量，这些振动态仅延伸到500 - 1000 cm，远低于约4000 cm的HF伸缩基频。因此，该方法在计算具有更多振动自由度的复合物方面具有特别的前景。
