Calculation of Accurate Thermo Chemical Data for Fluxional Molecules

热化学平衡和动力学数据是设计工艺、分子和催化剂所必需的。近几十年来，改进的算法和计算机能力极大地减少了电子能量计算中的不确定性。因此，对于柔性分子的自由能的计算，以及其所有平衡和动力学性质的计算，熵现在往往是最大的不确定源。这是因为，即使使用高精度的耦合团簇方法来模拟电子能量，仍然经常使用核运动的刚性转子谐振子(RRHO)模型来计算熵。PI小组开发了他们自己的计算柔性分子自由能的初始模型，在软件中实现了该模型的早期版本，并用小分子的实验数据验证了该模型，该早期版本对这些小分子是实用的。这里提出的项目旨在解决两个主要挑战，以便将该方法应用于更大的、技术相关的分子和过程：1)通过提高对该方法至关重要的蒙特卡罗积分步骤的效率，通过开发一种新的方法来在递归分层抽样方案下分配抽样预算，以及2)通过将相空间划分为近似调和自由度和完全非调和自由度，并采用混合方法来求解每个划分，以在总体精度损失最小的情况下降低问题的维度。该项目的预期结果将是一种经过验证的方法，用于准确计算技术上相关物质的热化学数据，这些数据可在以后用于设计新材料和工艺。

Thermochemical equilibrium and kinetic data are needed for the design of processes, molecules, and catalysts. In recent decades, improved algorithms and computer power have drastically reduced the uncertainties in the calculations of electronic energies. As a result, for the calculation of free energies of flexible molecules and, consequently, of all their equilibrium and kinetic properties, entropy is now often the largest source of uncertainty. This is because entropy is often still calculated with the rigid-rotor harmonic oscillator (RRHO) model for nuclear motion, even when highly accurate coupled-cluster methods are used to model electronic energies. The group of the PI have developed their own initial model for calculating the free energies of flexible molecules, have implemented an early version of it in software, and have validated it with experimental data for the small molecules for which this early version is practical. The here proposed project aims to solve two major challenges in order to apply the approach to larger, technically relevant molecules and processes: 1) by increasing the efficiency of the Monte Carlo integration step crucial to the approach by developing a new way to distribute sampling budgets under a recursive stratified sampling scheme, which we have derived recently and 2) by partitioning phase space into approximately harmonic degrees of freedom and fully anharmonic degrees of freedom and hybridizing methods to solve each partition to reduce the dimensionality of the problem with minimal loss in overall accuracy. The anticipated result of the project will be a validated method for the accurate calculation of thermochemical data of technically relevant substances that can later be used to design new materials and processes.