When the excitation energies of an ensemble of geometries, computed by the complete active space self‐consistent field (CASSCF) or other CAS‐based methods, are convoluted to obtain the absorption spectrum, it is advisable that all the considered geometries have the same molecular orbitals within the active space. In this work, we present an algorithm that evaluates the molecular orbital overlap matrix between a previously selected reference geometry, with the desired active space, and each of the sampled geometries. Based on the value of the overlap matrix elements, the algorithm determines whether one or more pairs of molecular orbitals of the sampled geometry have to be swapped for a subsequent CASSCF calculation. We have applied the developed algorithm to CASSCF and CASPT2 calculations for sets of geometries of the five canonical nucleobases in vacuum and in aqueous solvent. The algorithm shows a very good efficacy since it recovered the correct active space for 90% of the geometries which presented undesired molecular orbitals in the active space after the first CASSCF wavefunction optimization. In addition, the importance of having the same orbitals within the active space for all the geometries is discussed based on the corrected and uncorrected density of states for the nucleobases.

中文翻译：

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基于几何集合的多尺度QM / MM计算中的CASSCF活动空间校正算法

当通过完整的活动空间自洽场（CASSCF）或其他基于CAS的方法计算的一组几何体的激发能被卷积以获得吸收光谱时，建议所有考虑的几何体具有相同的分子活动空间内的轨道。在这项工作中，我们提出了一种算法，该算法可以评估先前选择的具有期望有效空间的参考几何图形与每个采样几何图形之间的分子轨道重叠矩阵。基于重叠矩阵元素的值，该算法确定是否必须交换一对或多对采样几何形状的分子轨道才能进行后续的CASSCF计算。我们已将开发的算法应用于CASSCF和CASPT2计算中的五个典型核碱基在真空和水性溶剂中的几何形状。该算法显示出非常好的功效，因为它在90％的几何形状中恢复了正确的有效空间，这在第一次CASSCF波函数优化之后在有效空间中显示了不需要的分子轨道。另外，基于核碱基的状态的校正和未校正的状态密度，讨论了对于所有几何结构，在活动空间内具有相同轨道的重要性。