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[交流] 如何选择势能模型？

那位高手能够介绍一下MS中CASTEP的势能模型的适用范围？
比如说LDA的（PWC,VWN），GGA的（PW91,BP,PBE,BLYP,BOP,VWN-BP,PRBE,HCTH），能够简述一下其大致差别更好

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1楼 2008-01-03 09:53:54

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谢谢，应该是这个吧

VWN: The Vosko-Wilk-Nusair (VWN) functional is the most popular LSD correlation potential. It uses a fit to accurate numerical results (by Ceperly and Alder) of a uniform electron gas. Ceperley and Alder performed quantum Monte Carlo calculations on a uniform electron gas at low and high spin limits for several electron densities. VWN uses the Pade interpolation procedure to fit the CA results for both the para and ferro states and for low and high densities. DMol3 uses the best VWN (so called "Fit"

parameters.

PWC: The Perdew-Wang (PWC) functional is a recent parameterization of the Ceperley and Alder data, which corrects some VWN problems with fitting. PWC is the default functional for DMol3 calculations.

The local spin-density (LSD) approximation accurately predicts structures, vibrations, and relative energies of covalent systems; however, bond energies are seriously overestimated. The local DFT should not be used for systems with weak bonds, such as hydrogen bonds. These problems with the LSD method can be corrected to a large extent by using the so-called gradient-corrected (or nonlocal) functionals.

P91, BP, BLYP, BOP: DMol3 supports several nonlocal exchange and correlation functionals. The most popular, the Becke exchange functional (B88) is used in conjunction with the Perdew-Wang correlation functional (BP) or the Lee-Yang-Parr correlation functional (BLYP). The so-called generalized gradient corrected (GGA) functional, by Perdew and Wang (P91) was derived by considering low and high density regimes and by enforcing various summation rules.

PBE: The PBE (Perdew, Burke and Enzerhof) functional (1996, 1997) is another GGA functional in which all the parameters (other then those in its LDA component) are fundamental constants. The exchange part of this functional is similar to the Becke formula (1986), and the correlation part is close to the Perdew-Wang functional (1986).This functional has a strong physical background, reliable numerical performance and it is frequently used in DFT calculations.

RPBE: More recently, Hammer, Hansen and Norskov (1999) proposed a modified version of the PBE formula that improves considerably thermochemical results. So far this functional, called RPBE, has been mainly used for solids.

HCTH: The HCTH functional, named for the authors' initials (Hamprecht et al., 1998), represents a "pragmatic" philosophy in designing a DFT functional. Assuming that the exact functional will never be found, they propose a flexible form of gradient corrected functional that is fitted to the training set of molecules. The default for the current implementation, the so called HCTH/407 functional, that was obtained by fitting to the set of the 407 atomic and molecular systems (Boese and Handy, 2001). This functional was found to predict a much improved thermochemistry for inorganic and hydrogen bonded systems. The standalone keyword for this functional is hcth407. The original HCTH-type functionals can be activated by keyword hcth93 (Hamprecht et al, 1998) and hcth147 (Boese et al. 2000).

Although the NLSD methods are significantly better than the LSD method, particularly in studying chemical reactions, the NLSD methods may still lead to reaction barriers that are too low.

VWN-BP: The VWN-BP functional is recommended for COSMO and COSMO-RS studies.

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7楼2008-01-04 12:18:32

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我看好你哦！

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LDA局域密度近似（LDA）：局域密度近似（LDA）是第一阶梯。它仅仅采用空间点r处的电子密度n（r）来决定那点交换-相关能密度的形式。交换-相关能密度由密度相同的均匀电子气完全确定。泛函的交换部分就准确的用均匀电子气的微分表达。各种不同的局域密度近似（LDA）仅仅是相关部分表示方法不同，所有现代应用的局域密度泛函都基于Ceperly和Alder`s在８０年代对均匀电子气总能量的Monte Carlo模拟。

广义梯度近似（ＧＧＡ）：ＧＧＡ是Jacob阶梯的第二个台阶，将电子密度的梯度也作为一个独立的变量（|∇n(r)|），在描述交换－相关能方面，梯度引入了非定域性。ＧＧＡ泛函包含了两个主要的方向：一个称为“无参数”，泛函中新的参数通过已知形式中参数或在其它准确理论帮助下得到。另外一个就是经验方法，未知参数来自于对实验数据的拟和或通过对原子和分子性质准确的计算。Perdew,Burke and Emzerhof(PBE)以及Perdew-Wang from 1991(PW91)是无参数的，在量子化学中广泛采用的GGA，比如Becke,Lee,Parr and Yang(BLYP)是经验性。LYP校正采用了密度的二阶Laplace算符，因此严格上讲属于Jacob阶梯的第三阶，但通常仍然归类为GGA.

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2楼2008-01-03 11:13:07

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