Department of Inorganic and Physical Chemistry, Ghent University, Krijgslaan 281 S3, 9000 Ghent, Belgium.
J Chem Phys. 2010 Mar 21;132(11):114112. doi: 10.1063/1.3354910.
A variational optimization of the second-order density matrix under the P-, Q-, and G-conditions was carried out for a set of diatomic 14-electron molecules, including N(2), O(2) (2+), NO(+), CO, and CN(-). The dissociation of these molecules is studied by analyzing several chemical properties (dipole moments, population analysis, and bond indices) up to the dissociation limit (10 and 20 A). Serious chemical flaws are observed for the heteronuclear diatomics in the dissociation limit. A careful examination of the chemical properties reveals that the origin of the dissociation problem lies in the flawed description of fractionally occupied species under the P-, Q-, and G-conditions. A novel constraint is introduced that imposes the correct dissociation and enforces size consistency. The effect of this constraint is illustrated with calculations on NO(+), CO, CN(-), N(2), and O(2)(2+).
对一组包括 N(2)、O(2)(2+)、NO(+)、CO 和 CN(-)在内的双原子 14 电子分子，在 P、Q 和 G 条件下进行了二阶密度矩阵的变分优化。通过分析几个化学性质（偶极矩、布居分析和键指数），直至达到离解极限（10 和 20 A），研究了这些分子的离解。在离解极限处，观察到异核双原子存在严重的化学缺陷。对化学性质的仔细检查表明，离解问题的根源在于 P、Q 和 G 条件下对分数占据物种的描述存在缺陷。引入了一个新的约束条件，该条件强制正确的离解并保持尺寸一致性。通过对 NO(+)、CO、CN(-)、N(2)和 O(2)(2+)的计算，说明了该约束条件的效果。
