优化几何构型对高级别从头算能量的影响

High-level Ab Initio Energy Divergences between Theoretical Optimized and Experimental Geometries

对《ＣＲＣ物理与化学手册》（第７７版）中第三周期以前的无机双原子分子，当其理论优化构型的相对误差大于２％时，分别在实验构型和最大偏差的理论构型下，计算了ＱＣＩＳＤ（Ｔ）／６－３１１＋Ｇ（３ｄｆ，２ｐ）能量并作了比较．结果表明，大多数能量的差别在４．２ｋＪ·ｍｏｌ^（－１）以内．由此说明，目前一般采用的构型优化理论方法，多数情况下不至于明显影响单点高级别从头算的计算精度．同时还发现，Ｇ２（ＱＣＩ）的高级从头算方法 ＱＣＩＳＤ（Ｔ）／６－３１１＋Ｇ（３ｄｆ，２ｐ），在实验构型下，并未给出最低的分子能量，ＭＰ２（ｆｕｌｌ）构型优化方法未能给出优化键长普遍的好结果，特别是对ＣＮ、ＣＰ和ＳＩＮ这种“４－５价电子结构”分子的偏差随基组增大而增大，导致较大的单点能量偏差．ＭＰ２（ｆｕｌｌ）／６－３１Ｇ（ｄ， ｐ）优化Ｏ_２和Ｎ_２的键长误差较大，导致单点能量偏差接近４，２ ｋＪ·ｍｏｌ^（－１），而ＭＰ２（ｆｕｌｌ）／６－３１１Ｇ（ｄ，ｐ）则较好地改善了它们的构型和ＱＣＩＳＤ（Ｔ）／６－３１１＋Ｇ（３ｄｆ，２ｐ）能量．ＢＮ（^（３）∏）的实验键长 １２８． １ｐｍ可能有误，有待进一步测定．

High-level ab initio, QCISD (T) /6-311 + G(3 df, 2p), energy from the least accurate theoretical bond distance was calculated and compared with that from the experimental geometry on each of the thirty four diatomic molecules having a relative error (ropt. - rexp. ) / rexp. of theoretical bond length greater than 2%. These molecules were chosen from all of the sixty first- and second-row diatomic inorganic species collected in the 77th CRC Handbook of Chemistry and Physics involved in our previous comparison of systematic geometry optimizations. It was found, unexpectedly, that QCISD(T) /6-311 + G (3 df, 2p) calculations did not result in the lowest energy with the experimental bond distance in a few cases, which implies that QCISD(T)/6-311 + G(3df, 2p) method has some deficiencies in the point of view of 'high-level ab initio'. Most of the QCISD . (T) /6-311 + G(3 df, 2p) energy divergences between the two different geometries were less than 4. 2 kJ. mol-1 with six exceptions with BN, CN, CP, N_2, O_2 and SiN molecules and especially on CN radical, Which resulted in an error as large as 15.8 kJ· mol-1 at the MP2 (full) /6-311G(2 d, p) geometry. For CN, CP and SiN radicals, the energy errors were also from the poor geometry of MP2 optindzations. It was also found that the larger the basis sets used the poorer the geometries were for these three radicals at MP2 level. For the ground state of BN(^(3)∏) radical, the experimental bond length 128. 1 pm maybe in doubt and has to be re-examined. For N_2 and O_2, MP2(full) gradient optilnization at the basis sets of valence triple-, 6-311G (d, p ), improved the geometries as well as the high-level QCISD(T) energies compared with what at the valence double-, 6-31G(d, p), basis sets.