Neumann Marcus A, Perrin Marc-Antoine
Avant-garde Materials Simulation SARL, 30 bis rue du Vieil Abreuvoir, 78100 Saint Germain en Laye, France.
J Phys Chem B. 2005 Aug 18;109(32):15531-41. doi: 10.1021/jp050121r.
By combination of high level density functional theory (DFT) calculations with an empirical van der Waals correction, a hybrid method has been designed and parametrized that provides unprecedented accuracy for the structure optimization and the energy ranking of molecular crystals. All DFT calculations are carried out using the VASP program. The van der Waals correction is expressed as the sum over atom-atom pair potentials with each pair potential for two atoms A and B being the product of an asymptotic C(6,A,B)/r(6) term and a damping function d(A,B)(r). Empirical parameters are provided for the elements H, C, N, O, F, Cl, and S. Following Wu and Yang, the C(6) coefficients have been determined by least-squares fitting to molecular C(6) coefficients derived by Meath and co-workers from dipole oscillator strength distributions. The damping functions d(A,B)(r) guarantee the crossover from the asymptotic C(6,A,B)/r(6) behavior at large interatomic distances to a constant interaction energy at short distances. The careful parametrization of the damping functions is of crucial importance to obtain the correct balance between the DFT part of the lattice energy and the contribution from the empirical van der Waals correction. The damping functions have been adjusted to yield the best possible agreement between the unit cells of a set of experimental low temperature crystal structures and their counterparts obtained by lattice energy optimization using the hybrid method. On average, the experimental and the calculated unit cell lengths deviate by 1%. To assess the performance of the hybrid method with respect to the lattice energy ranking of molecular crystals, various crystal packings of ethane, ethylene, acetylene, methanol, acetic acid, and urea have been generated with Accelrys' Polymorph Predictor in a first step and optimized with the hybrid method in a second step. In five out of six cases, the experimentally observed low-temperature crystal structure corresponds to the most stable calculated structure.
通过将高水平密度泛函理论（DFT）计算与经验性范德华修正相结合，设计并参数化了一种混合方法，该方法在分子晶体的结构优化和能量排序方面提供了前所未有的准确性。所有DFT计算均使用VASP程序进行。范德华修正表示为原子 - 原子对势的总和，两个原子A和B的每对势是渐近C(6,A,B)/r(6)项与阻尼函数d(A,B)(r)的乘积。为元素H、C、N、O、F、Cl和S提供了经验参数。按照吴和杨的方法，C(6)系数通过对Meath及其同事从偶极振子强度分布推导的分子C(6)系数进行最小二乘拟合来确定。阻尼函数d(A,B)(r)保证了在大原子间距时从渐近C(6,A,B)/r(6)行为到短距离时恒定相互作用能的转变。阻尼函数的精确参数化对于在晶格能量的DFT部分和经验性范德华修正的贡献之间获得正确平衡至关重要。已调整阻尼函数，以使一组实验低温晶体结构的晶胞与其使用混合方法通过晶格能量优化获得的对应晶胞之间达成尽可能好的一致性。平均而言，实验和计算得到的晶胞长度偏差为1%。为了评估混合方法在分子晶体晶格能量排序方面的性能，第一步使用Accelrys的多晶型预测器生成了乙烷、乙烯、乙炔、甲醇、乙酸和尿素的各种晶体堆积，第二步使用混合方法进行优化。在六个案例中的五个案例中，实验观察到的低温晶体结构对应于计算得到的最稳定结构。
