The implementation of the cyclic cluster model (CCM) for molecular mechanics is presented in the framework of the computational chemistry program deMon2k. Because the CCM is particularly well-suited for the description of periodic systems with defects, it can be used for periodic QM/MM approaches where the non-periodic QM part is treated as a defect in a periodic MM surrounding. To this end, we present here the explicit formulae for the evaluation of the Ewald sum and its first- and second-order derivatives as implemented in deMon2k. The outlined implementation was tested in molecular dynamics (MD) simulations and periodic structure optimization calculations. MD simulations of an argon system were carried out using the Nosé-Hoover chain (NHC) thermostat and the Martyna-Tobias-Klein (MTK) barostat to control the temperature and pressure of the system, respectively. For the validation of CCM structure optimization a set of molecular crystals were optimized using the Ewald method for the evaluation of the electrostatic interactions. Two optimization procedures for the determination of the atomic positions and CCM cell parameters were tested. Our results show that the simultaneous optimization of the atomic positions and cell parameters is most efficient.

中文翻译：

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循环簇模型的分子力学实现

分子力学循环簇模型（CCM）的实现在计算化学程序的框架中提出恶魔2k。由于 CCM 特别适合描述有缺陷的周期系统，因此它可用于周期 QM/MM 方法，其中非周期 QM 部分被视为周期 MM 周围的缺陷。为此，我们在这里提出了评估 Ewald 和及其一阶和二阶导数的显式公式，如恶魔2k。概述的实现在分子动力学（MD）模拟和周期性结构优化计算中进行了测试。使用 Nosé-Hoover 链 (NHC) 恒温器和 Martyna-Tobias-Klein (MTK) 恒压器分别控制系统的温度和压力，对氩气系统进行 MD 模拟。为了验证 CCM 结构优化，使用 Ewald 方法优化了一组分子晶体，以评估静电相互作用。测试了确定原子位置和 CCM 单元参数的两种优化程序。我们的结果表明，原子位置和晶胞参数的同时优化是最有效的。