The failure of Hartree-Fock models

The failure of Hartree-Fock models to properly account for the energetics of reactions involving bond making and bond breaking,and probably as well for the geometries of transition-metal compounds, can be traced to incomplete description of the coupling f motions of electrons(electron correlation"). A number of correlated models, among the most popular, the MP2 modelso, have been developed. These generally provide descriptions of equilibrium and transition-state and conformations, as well as reaction thermodynamics and kinetics. including reactions where bonds are broken and formed. Correlated calculations such as MP2 are, however, much more costly than alculations, and are usually not practical for molecules with more than 20 atoms.

An alternative approach to the problem of electron correlation is found in so-called density functional models". Instead of approximating the true many-electron solution by a composite of one-electron solutions, density functional models account explicitely for many- electron effects by"building in" a correlation term based on an idealized" many-electron problem. Because many-electron effects are taken into account explicitly, the results of a density functional calculation should be superior to those from a Hartree-Fock treatment, and comparable to those from a"conventional" correlated calculation, such as an MP2 calculation. Even so, the computational costinvolved in actually doing a density functional calculation actually increases more slowly with increasing molecular size than the cost of aHartree- Fock or MP2 calculation. There are significant drawbacks with density functional models. Improvement of Hartree-Fock and correlated models such as MP2 follows in a straightforward manner by"easing" the approximations originally made to solve the correctmany electron problem. However, the only way that density functional models may be improved to bring the"idealized" many-electron problem, on which the correlation term is based, closer to the"real" problem. Unfortunately, it is not yet obvious how to do this. Additionally density functional models are not presently amenable to fully analytical treatments(as are Hartree-Fock and MP2 models). Required numerical integration steps introduce uncertainties into the calculations, and these need to be carefully monitored and controlled.


This brief guide focuses on the application of molecular mechanics semi-empirical and Hartree-Fock molecular orbital models,

結果 (繁體中文) 1:

對失敗的導論模型正確帳戶的反應涉及債券和債券打破，以及可能為過渡金屬化合物的幾何尺寸，可以追溯到不完整描述電子 （電子相關"） 耦合 f 運動的能量。已開發了大量的相關模型，最受歡迎，MP2 modelso。這些一般提供平衡和過渡態和構象，以及反應熱力學和動力學的描述。包括地方債券的破裂和形成的反應。如 MP2 相關的計算是，然而，比 alculations，並對有 20 多個原子的分子通常並不實用的昂貴得多。電子相關問題的替代方法是在發現所謂密度函數模型"。而不是由一個電子解決方案的組合逼近真實的多電子解決方案，密度函數模型考慮以多電子效果所明確"建設在"相關術語基於理想化"多電子問題。因為許多電子效應都明確考慮在內，密度函數計算的結果應該是優於導論治療，和那些從"常規"的相關計算，如 MP2 計算相媲美。即便如此，在實際上做密度函數計算的計算 costinvolved 實際上更加緩慢地增加隨著分子大小比 aHartree Fock 或 MP2 計算成本的增加。是有重大缺陷的密度函數模型。改進的導論和相關的模型，如 MP2 遵循直截了當地由"寬鬆"最初作出解決 correctmany 電子問題的逼近。然而，可能提高密度函數模型，為使這種"理想化"的多電子問題，唯一方法的相關術語是基礎，更接近于"真實"的問題。不幸的是，它並沒有明顯如何做到這一點。另外密度函數模型並非目前服從全解析治療 （因為導論和 MP2 模型）。所需的數值集成步驟引入計算和這些需要仔細監測和控制的不確定性。這個簡短的指南重點介紹的半經驗分子力學和導論分子軌道模型，應用

結果 (繁體中文) 2:

哈特里-福克模型來正確地考慮涉及用於過渡金屬化合物的幾何鍵接通和鍵斷裂，並可能還有反應的熱力學的失敗，可以追溯到電子的耦合˚F運動的不完全的描述（電子相關“）。已經制定了許多相關的車型，當中最流行 ​​，MP2 modelso，這些一般提供平衡和過渡態和構象的描述，以及反應的熱力學和動力學。包括反應，其中鍵斷裂和形成。相關的計算，如MP2，然而，比alculations更加昂貴，而且通常是不切合實際的超過20個原子的分子。

另一種方法，以電子相關的問題，所謂的密度功能模型發現“ 。代替由單電子解決方案的複合近似真多電子溶液，密度泛函模型明確地考慮由“建築”的基礎上的理想化“多電子問題的相關術語多對一電子的影響。因為許多電子效果考慮明確地，一個密度泛函計算的結果要優於那些從哈特里-福克處理，並與那些從“傳統的”相關的計算，如MP2計算。即使這樣，計算costinvolved在實際做一個密度泛函計算實際上比aHartree-福克或MP2計算的成本增加分子體積增大更慢。沒有與密度功能模型哈特里-福克的。改進和相關模型顯著缺點，如MP2直截了當如下方式由“寬鬆”最初提出的近似解決correctmany電子問題，但唯一的辦法是密度泛函模型可以提高帶來的“理想化”多電子的問題，對其中的相關術語的基礎，更接近“真正的”問題。不幸的是，它是尚未明顯如何做到這一點。此外密度泛函模型並不適合目前全面分析處理（因為是哈特里-福克和MP2型號）。所需數值積分步驟不確定性帶入計算，而這些都需要仔細監測和控制。


這份簡短的指南重點介紹了分子力學半經驗和哈特利-福克分子軌道模型的應用，