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小木虫论坛-学术科研互动平台 » 计算模拟区 » 量子化学 » Gaussian » 【求助】高斯CASSCF的mannual和方法
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1楼 2010-05-17 10:12:14
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aylayl08(金币+2):谢谢讨论 2010-05-17 18:26:53
There is no unique answer to this question. The major problem is the limitation of the active space to 12-15 orbitals enforced by the method. In most cases this is no problem and the choice of active orbitals is obvious (for example the pi-orbitals in a conjugated system). The answer depends strongly on the problem under consideration. If it is a GS energy surface one should be careful to include all orbitals the occupation number of which may vary considerable along the path(s) followed on the surface. In excited states studies one need to include enough orbitals to describe the excited states. This often means both valence and Rydberg type orbitals.
In difficult cases it might be advantageous by starting a study with a RASSCF calculation where RAS2 only includes the open shell orbitals (if any), RAS1 the orbitals that can possibly be interesting as active, and RAS3 the corresponding correlating orbitals (often one just chooses all valence orbitals). By allowing 2-4 electrons in RAS3 one can get quite a good description of the near degeneracies. The result can be used to choose an active space for a CASSCF/CASPT2 calculation. Note that this test calculation may have to be performed on several places on an energy surface. In CASSCF calculations one should preferably use the same active space over the whole surface.
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2楼2010-05-17 11:25:11
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