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arXiv 2012.08306physics.chem-ph
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约束和非约束Hartree-Fock和Kohn-Sham方法的信赖域增广Hessian实现

A trust-region augmented Hessian implementation for restricted and unrestricted Hartree-Fock and Kohn-Sham methods

Benjamin Helmich-Paris

摘要:

我们提出了一个信赖域增广海森实现(TRAH-SCF)的限制和非限制的Hartree-Fock和Kohn-Sham方法。使用TRAH-SCF,收敛总是可以通过严格的收敛阈值来实现,这只需要适度的迭代次数。我们的收敛基准研究和我们的说明性应用集中在开壳层分子,反铁磁耦合系统,它是出了名的复杂收敛的Roothaan-Hall自洽场(SCF)方程。我们比较了TRAH迭代次数达到收敛与Pulay的和Kolmar的(K)变种的直接反演的迭代子空间(DIIS)方法,并分析了所获得的SCF解决方案。通常TRAH-SCF找到一个能量比DIIS和KDIIS低的解。对于不受限制的计算,这伴随着较大的自旋污染,即与期望的自旋限制<S^2>期望值的较大偏差。然而,也有少数情况下,DIIS找到了比KDIIS和TRAH能量更低的解决方案。在极少数情况下,TRAH-SCF和KDIIS也可能收敛到激发态行列式解。对于那些负间隙TRAH解决方案的计算,标准DIIS总是发散。如果可以比较的话,TRAH通常需要比DIIS和KDIIS更多的迭代来收敛,因为对于每一组新的轨道,水平位移的牛顿-拉夫森方程都是通过本征值问题近似迭代求解的。尽管如此,TRAH-SCF的总运行时间仍然具有竞争力的DIIS为基础的方法,即使采用扩展的基集,这是说明了一个大的血蓝蛋白模型复杂。

英文摘要:

We present a trust-region augmented Hessian implementation (TRAH-SCF) for restricted and unrestricted Hartree-Fock and Kohn-Sham methods. With TRAH-SCF convergence can always be achieved with tight convergence thresholds, which requires just a modest number of iterations. Our convergence benchmark study and our illustrative applications focus on open-shell molecules, also antiferromagnetically coupled systems, for which it is notoriously complicated to converge the Roothaan-Hall self-consistent field (SCF) equations. We compare the number of TRAH iterations to reach convergence with those of Pulay's and Kolmar's (K) variant of the direct inversion of the iterative subspace (DIIS) method and also analyze the obtained SCF solutions. Often TRAH-SCF finds a symmetry-broken solution with a lower energy than DIIS and KDIIS. For unrestricted calculations, this is accompanied by a larger spin contamination, i.e. larger deviation from the desired spin-restricted <S^2> expectation value. However, there are also rare cases in which DIIS finds a solution with a lower energy than KDIIS and TRAH. In rare cases, both TRAH-SCF and KDIIS may also converge to an excited-state determinant solution. For those calculations with negative-gap TRAH solutions, standard DIIS always diverges. If comparable, TRAH usually needs more iterations to converge than DIIS and KDIIS because for every new set of orbitals the level-shifted Newton-Raphson equations are solved approximately and iteratively by means of an eigenvalue problem. Nevertheless, the total runtime of TRAH-SCF is still competitive with the DIIS-based approaches even if extended basis sets are employed, which is illustrated for a large hemocyanin model complex.

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