Chemical Physics Department, Weizmann Institute of Science, 76100, Rehovoth, Israel.
J Chem Phys. 2011 Jun 21;134(23):234305. doi: 10.1063/1.3599053.
Internal conversion is an inherently quantum mechanical process. To date, "on the fly" computation of internal conversion rates is limited to harmonic approximations, which would seem to be especially unsuitable, given that the typical transition to the ground electronic state occurs at energies which are far from the harmonic limit. It is thus of interest to study the applicability of the semiclassial initial value representation (SCIVR) approach which is in principle amenable to on the fly studies even with "many" degrees of freedom. In this paper we study the applicability of the Herman-Kluk (HK) SCIVR to a model system with two coupled and anharmonic degrees of freedom. We find that (a) the HK SCIVR is a good approximation to the exact quantum dynamics; (b) computation of the first order correction to the HK-SCIVR approximation corroborates the accuracy; (c) by studying a large parameter range, we find that the harmonic approximation is mostly unsatisfactory; and (d) for the specific model used, the coupling between the modes was found to be relatively unimportant. These results imply that the HK-SCIVR methodology is a good candidate for on the fly studies of internal conversion processes of "large" molecules.
内转换是一种固有的量子力学过程。迄今为止，“实时”计算内转换速率仅限于谐波近似，这似乎特别不合适，因为典型的向基态电子跃迁发生在远非谐波极限的能量处。因此，研究半经典初始值表示（SCIVR）方法的适用性很有意义，该方法原则上即使有“许多”自由度也适用于实时研究。在本文中，我们研究了具有两个耦合非谐自由度的模型系统对赫尔曼-克鲁克（HK）SCIVR 的适用性。我们发现：(a) HK-SCIVR 是对精确量子动力学的良好近似；(b) 对 HK-SCIVR 近似的一阶修正的计算证实了其准确性；(c) 通过研究大范围参数，我们发现谐波近似大多不令人满意；(d) 对于所使用的特定模型，发现模式之间的耦合相对不重要。这些结果表明，HK-SCIVR 方法是实时研究“大”分子内转换过程的一个很好的候选方法。
