Tangney Paul
International School for Advanced Studies, via Beirut 2-4, 34013 Trieste, Italy.
J Chem Phys. 2006 Jan 28;124(4):044111. doi: 10.1063/1.2162893.
The theory underlying the Car-Parrinello extended-Lagrangian approach to ab initio molecular dynamics (CPMD) is reviewed and reexamined using "heavy" ice as a test system. It is emphasized that the adiabatic decoupling in CPMD is not a decoupling of electronic orbitals from the ions but only a decoupling of a subset of the orbital vibrational modes from the rest of the necessarily coupled system of orbitals and ions. Recent work [J. Chem. Phys. 116, 14 (2002)] has pointed out that, due to the orbital-ion coupling that remains once adiabatic decoupling has been achieved, a large value of the fictitious mass mu can lead to systematic errors in the computed forces in CPMD. These errors are further investigated in the present work with a focus on those parts of these errors that are not corrected simply by rescaling the masses of the ions. It is suggested that any comparison of the efficiencies of Born-Oppenheimer molecular dynamics (BOMD) and CPMD should be performed at a similar level of accuracy. If accuracy is judged according to the average magnitude of the systematic errors in the computed forces, the efficiency of BOMD compares more favorably to that of CPMD than previous comparisons have suggested.
以“重”冰作为测试系统，对从头算分子动力学（CPMD）的Car-Parrinello扩展拉格朗日方法的基础理论进行了回顾和重新审视。需要强调的是，CPMD中的绝热解耦并非电子轨道与离子的解耦，而仅仅是一部分轨道振动模式与其余必然耦合的轨道和离子系统的解耦。最近的工作[《化学物理杂志》116, 14 (2002)]指出，由于绝热解耦实现后仍存在的轨道-离子耦合，虚拟质量μ的较大值会导致CPMD中计算力的系统误差。在本工作中进一步研究了这些误差，重点关注那些不能简单通过重新缩放离子质量来校正的误差部分。建议对玻恩-奥本海默分子动力学（BOMD）和CPMD的效率进行任何比较时，都应在相似的精度水平上进行。如果根据计算力中系统误差的平均大小来判断精度，那么BOMD的效率与CPMD相比，比之前的比较结果更有利。
