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Towards multireference equivalents of the G2 and G3 methods

机译：迈向G2和G3方法的多引用等效项

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The effect of replacing the standard single-determinant reference wave functions in variants of G2 and G3 theory by multireference (MR) wave functions based on a full-valence complete active space has been investigated. Twelve methods of this type have been introduced and comparisons, based on a slightly reduced G2-1 test set, are made both internally and with the equivalent single-reference methods. We use CASPT2 as the standard MR-MP2 method and MRCl+Q as the higher correlation procedure in these calculations. We find that MR-G2(MP2,SVP), MR-G2(MP2), and MR-G3(MP2) perform comparably with their single-reference analogs, G2(MP2,SVP), G2(MP2), and G3(MP2), with mean absolute deviations (MADs) from the experimental data of 1.41, 1.54, and 1.23 kcal mol(-1), compared with 1.60, 1.59, and 1.19 kcal mol(-1), respectively. The additivity assumptions in the MR-Gn methods have been tested by carrying out MR-G2/MRCI+Q and MR-G3/MRCI+Q calculations, which correspond to large-basis-set MRCI+Q+ZPVE+HLC calculations. These give MADs of 1.84 and 1.58 kcal mol(-1), respectively, i.e., the agreement with experiment is somewhat worse than that obtained with the MR-G2(MP2) and MR-G3(MP2) methods. In a third series of calculations, we have examined pure MP2 and MR-MP2 analogs of the G2 and G3 procedures by carrying out large-basis-set MP2 and CASPT2(+ZPVE+HLC) calculations. The resultant methods, which we denote G2/MP2, G3/MP2, MR-G2/MP2, and MR-G3/MP2, give MADs of 4.19, 3.36, 2.01, and 1.66 kcal mol(-1), respectively. Finally, we have examined the effect of using MCQDPT2 in place of CASPT2 in five of our MR-Gn procedures, and find that there is a small but consistent deterioration in performance. Our calculations suggest that the MR-G3(MP2) and MR-G3/MP2 procedures may be useful in situations where a multireference approach is desirable. (C) 2001 American Institute of Physics. [References: 49]
机译：研究了基于全价完全活动空间的多参考（MR）波函数替换G2和G3理论变体中的标准单行参考波函数的效果。已经介绍了这种类型的十二种方法，并且在内部和等效单参考方法的基础上，基于略有减少的G2-1测试集进行了比较。在这些计算中，我们使用CASPT2作为标准的MR-MP2方法，并使用MRCl + Q作为更高的相关程序。我们发现MR-G2（MP2，SVP），MR-G2（MP2）和MR-G3（MP2）与它们的单参考类似物G2（MP2，SVP），G2（MP2）和G3（ MP2），与实验数据的平均绝对偏差（MAD）为1.41、1.54和1.23 kcal mol（-1），而分别为1.60、1.59和1.19 kcal mol（-1）。 MR-Gn方法中的可加性假设已通过执行MR-G2 / MRCI + Q和MR-G3 / MRCI + Q计算进行了测试，这与大基集MRCI + Q + ZPVE + HLC计算相对应。这些分别给出1.84和1.58 kcal mol（-1）的MAD，即与实验的一致性比MR-G2（MP2）和MR-G3（MP2）方法得到的差一些。在第三系列计算中，我们通过执行大基集MP2和CASPT2（+ ZPVE + HLC）计算，检查了G2和G3过程的纯MP2和MR-MP2类似物。所得方法分别表示G2 / MP2，G3 / MP2，MR-G2 / MP2和MR-G3 / MP2，其MAD分别为4.19、3.36、2.01和1.66 kcal mol（-1）。最后，我们在五个MR-Gn程序中检查了使用MCQDPT2代替CASPT2的效果，发现性能出现了微小但持续的下降。我们的计算表明，在需要多引用方法的情况下，MR-G3（MP2）和MR-G3 / MP2程序可能会有用。 （C）2001美国物理研究所。 [参考：49]

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