Ming Yi, Lai Geeling, Tong Chinghang, Wood Robert H, Doren Douglas J
Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, USA.
J Chem Phys. 2004 Jul 8;121(2):773-7. doi: 10.1063/1.1756574.
An efficient approach is described for using accurate ab initio calculations to determine the rates of elementary condensation and evaporation processes that lead to nucleation of aqueous aerosols. The feasibility of the method is demonstrated in an application to evaporation rates of water dimer at 230 K. The method, known as ABC-FEP (ab initio/classical free energy perturbation), begins with a calculation of the potential of mean force for the dissociation (evaporation) of small water clusters using a molecular dynamics (MD) simulation with a model potential. The free energy perturbation is used to calculate how changing from the model potential to a potential calculated from ab initio methods would alter the potential of mean force. The difference in free energy is the Boltzmann-weighted average of the difference between the ab initio and classical potential energies, with the average taken over a sample of configurations from the MD simulation. In principle, the method does not require a highly accurate model potential, though more accurate potentials require fewer configurations to achieve a small sampling error in the free energy perturbation step. To test the feasibility of obtaining accurate potentials of mean force from ab initio calculations at a modest number of configurations, the free energy perturbation method has been used to correct the errors when some standard models for bulk water (SPC, TIP4P, and TIP4PFQ) are applied to water dimer. To allow a thorough exploration of sampling issues, a highly accurate fit to results of accurate ab initio calculations, known as SAPT-5s, as been used a proxy for the ab initio calculations. It is shown that accurate values for a point on the potential of mean force can be obtained from any of the water models using ab initio calculations at only 50 configurations. Thus, this method allows accurate simulations of small clusters without the need to develop water models specifically for clusters.
本文描述了一种有效的方法，用于通过精确的从头计算来确定导致水气溶胶成核的基本凝聚和蒸发过程的速率。该方法在230K下水二聚体蒸发速率的应用中得到了验证。该方法被称为ABC-FEP（从头算/经典自由能微扰法），首先使用具有模型势的分子动力学（MD）模拟来计算小水团簇解离（蒸发）的平均力势。自由能微扰用于计算从模型势转变为从头算方法计算得到的势时，平均力势将如何变化。自由能的差异是从头算和经典势能差异的玻尔兹曼加权平均值，该平均值是在MD模拟的一组构型样本上取得的。原则上，该方法不需要非常精确的模型势，不过更精确的势在自由能微扰步骤中需要更少的构型就能实现较小的采样误差。为了测试在适度数量的构型下通过从头计算获得精确平均力势的可行性，当将一些用于 bulk water 的标准模型（SPC、TIP4P 和 TIP4PFQ）应用于水二聚体时，使用自由能微扰方法来校正误差。为了全面探讨采样问题，已使用一种对精确从头计算结果的高精度拟合，即 SAPT-5s，来替代从头计算。结果表明，仅使用50个构型的从头计算，就可以从任何水模型中获得平均力势上某一点的精确值。因此，该方法无需专门为团簇开发水模型，就能对小团簇进行精确模拟。
