Mannfors Berit, Palmo Kim, Krimm Samuel
LSA Biophysics, University of Michigan, 930 North University Avenue, Ann Arbor, Michigan 48109, USA.
J Phys Chem A. 2008 Dec 11;112(49):12667-78. doi: 10.1021/jp806948w.
Our ab initio transformed spectroscopically determined force field (SDFF) methodology emphasizes, in addition to accurate structure and energy performance, comparable prediction of vibrational properties in order to improve reproduction of interaction forces. It is now applied to the determination of a molecular mechanics (MM) force field for the water monomer and dimer as an initial step in developing a more physically based treatment of the hydrogen bonding that not only underlies condensed-phase water but also must be important in molecular-level protein-water interactions. Essential electrical components of the SDFF for monomer water are found to be the following: an off-plane charge distribution, this distribution consisting of four off-atom charge sites in traditional lone pair (LP) but also in inverted lone pair (ILP) positions; allowance for a diffuse size to these off-atom sites; and the incorporation of charge fluxes (i.e., the change in charge with change in internal coordinate). Parametrization of such an LP/ILP model together with the SDFF analytically transformed valence force field results in essentially exact agreement with ab initio (in this case MP2/6-31++G(d,p)) structure, electrical, and vibrational properties. Although we demonstrate that the properties of this monomer electrical model together with its van der Waals and polarization interactions are transferable to the dimer, this is not sufficient in reproducing comparable dimer properties, most notably the huge increase in infrared intensity of a donor OH stretch mode. This deficiency, which can be eliminated by a large dipole-derivative-determined change in the effective charge flux of the donor hydrogen-bonded OH bond, is not accounted for by the charge flux change in this bond due to the induction effects of the acceptor electric field alone, and can only be fully removed by an added bond flux associated with the extent of overlap of the wave functions of the two molecules. We show that this overlap charge flux (OCF) emulates an actual O-H...LP-O intermolecular dipole flux, reflecting the unitary nature of the hydrogen-bonded system in the context of MM-separable molecules. The effectiveness of incorporating the OCF noncanonical character demonstrates that a distinctively QM-unique property can be substantively represented in MM energy functions.
我们的从头算变换光谱确定力场（SDFF）方法，除了强调准确的结构和能量性能外，还注重对振动特性进行可比预测，以改进相互作用力的再现。目前，该方法被应用于确定水单体和二聚体的分子力学（MM）力场，这是开发更基于物理的氢键处理方法的第一步，这种氢键不仅是凝聚相水的基础，而且在分子水平的蛋白质 - 水相互作用中也必定起着重要作用。发现单体水的SDFF的基本电学成分如下：面外电荷分布，该分布由传统孤对（LP）以及反转孤对（ILP）位置上的四个离原子电荷位点组成；考虑这些离原子位点的弥散尺寸；以及纳入电荷通量（即，电荷随内部坐标变化而变化）。这种LP/ILP模型的参数化与SDFF解析变换价键力场一起，导致与从头算（在这种情况下为MP2/6 - 31++G(d,p)）结构、电学和振动特性基本完全一致。虽然我们证明了这种单体电学模型的性质及其范德华和极化相互作用可转移到二聚体，但这对于再现可比的二聚体性质是不够的，最显著的是供体OH伸缩模式的红外强度大幅增加。这种缺陷可以通过供体氢键合OH键的有效电荷通量中由大偶极导数确定的变化来消除，仅受体电场的诱导效应导致的该键电荷通量变化无法解释这一缺陷，并且只有通过与两个分子波函数重叠程度相关的附加键通量才能完全消除。我们表明，这种重叠电荷通量（OCF）模拟了实际的O - H...LP - O分子间偶极通量，反映了在MM可分离分子背景下氢键系统的整体性。纳入OCF非规范特征的有效性表明，一个独特的量子力学（QM）独特性质可以在MM能量函数中得到实质性体现。
