Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore.
J Chem Phys. 2011 Sep 14;135(10):104107. doi: 10.1063/1.3633273.
We have performed Car-Parrinello molecular dynamics (CPMD) calculations of the hydrogen-bonded NH(3)-HCl dimer. Our main aim is to establish how ionic-orbital coupling in CPMD affects the vibrational dynamics in hydrogen-bonded systems by characterizing the dependence of the calculated vibrational frequencies upon the orbital mass in the adiabatic limit of Car-Parrinello calculations. We use the example of the NH(3)-HCl dimer because of interest in its vibrational spectrum, in particular the magnitude of the frequency shift of the H-Cl stretch due to the anharmonic interactions when the hydrogen bond is formed. We find that an orbital mass of about 100 a.u. or smaller is required in order for the ion-orbital coupling to be linear in orbital mass, and the results for which can be accurately extrapolated to the adiabatic limit of zero orbital mass. We argue that this is general for hydrogen-bonded systems, suggesting that typical orbital mass values used in CPMD are too high to accurately describe vibrational dynamics in hydrogen-bonded systems. Our results also show that the usual application of a scaling factor to the CPMD frequencies to correct for the effects of orbital mass is not valid. For the dynamics of the dimer, we find that the H-Cl stretch and the N-H-Cl bend are significantly coupled, suggesting that it is important to include the latter degree of freedom in quantum dynamical calculations. Results from our calculations with deuterium-substitution show that both these degrees of freedom have significant anharmonic interactions. Our calculated frequency for the H-Cl stretch using the Becke-exchange Lee-Yang-Parr correlation functional compares reasonably well with a previous second-order Møller-Plesset calculation with anharmonic corrections, although it is low compared to the experimental value for the dimer trapped in a neon-matrix.
我们进行了 Car-Parrinello 分子动力学（CPMD）计算，研究了氢键 NH(3)-HCl 二聚体。我们的主要目的是通过在绝热极限下，通过 Car-Parrinello 计算中轨道质量的特征，来确定 CPMD 中的离子轨道耦合如何影响氢键系统中的振动动力学。我们使用 NH(3)-HCl 二聚体作为示例，因为其振动光谱具有研究价值，特别是当氢键形成时，由于非谐相互作用，H-Cl 伸缩的频率偏移幅度。我们发现，需要约 100 个 a.u.或更小的轨道质量，才能使离子轨道耦合在轨道质量上呈线性，并且可以准确外推到零轨道质量的绝热极限。我们认为这对于氢键系统是普遍的，这表明 CPMD 中通常使用的轨道质量值过高，无法准确描述氢键系统中的振动动力学。我们的结果还表明，通常对 CPMD 频率应用缩放因子以校正轨道质量的影响是无效的。对于二聚体的动力学，我们发现 H-Cl 伸缩和 N-H-Cl 弯曲明显耦合，这表明在量子动力学计算中包含后者自由度非常重要。我们对氘取代的计算结果表明，这两个自由度都具有显著的非谐相互作用。我们使用 Becke 交换 Lee-Yang-Parr 相关泛函计算的 H-Cl 伸缩频率与以前的带有非谐校正的二阶 Møller-Plesset 计算结果相当吻合，尽管与被困在氖基质中的二聚体的实验值相比，该值较低。
