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How to specify the spin-multiplicity?

已有 8055 次阅读 2013-5-18 21:32 |系统分类:科研笔记

在用gaussian时,如果分子的基态不是单重态,可以通过在title section里指定spin来计算其他组态如三重态的能量。

比如以下的计算指定氧原子为单重态
--------------------------
# hf/6-31G(d)

Oxygen atom

0 1
O 0.0000 0.0000 0.0000

--------------------------

而以下的输入则指定氧原子为三重态
--------------------------
# hf/6-31G(d)

Oxygen atom

0 3
O 0.0000 0.0000 0.0000

--------------------------

在VASP中,控制自旋的参数是INCAR中的ISPIN, ISPIN=1表示进行非自旋极化的计算,而ISPIN=2表示进行自旋极化的计算。为了进一步得到我们所需要的组态,可以通过NUPDOWN指定自旋向上和自旋向下的电子数差,此方法与直接指定自旋多重度是一样的。

VASP里,C的参考态是S2P2,即C的基态,ISPIN=2应该是这个态。

值得注意的是,在进行固定自旋多重态的计算中,如果初始的电荷密度的自旋极矩和INCAR中设置的NUPDOWN不同的话,收敛会很慢。如果是从读入的初始波函数开始计算,则没有类似问题。

以下是vasp手册上的原话:
There is a word of caution required: If NUPDOWN is set in the INCAR file the initial moment for the charge density should be the same. Otherwise convergence can slow down. When starting from atomic charge density (ICHARG=2), VASP will try to do this automatically by setting MAGMOM to NUPDOWN/NIONS. The user can of course overwrite this default by specifying a different MAGMOM (which should still result in the correct total moment). If one starts from the wavefunctions, the initial moment will be always correct, because VASP will ``push'' the required number of electrons from the down to the up component. If starting from a chargedensity supplied in the CHGCAR file (ICHARG=1), the initial moment is usually incorrect!

 

Hyperchem中自选多重度设置
在Setup设置中进行设置
Charge和Spin Multiplicity在Ab initio和Semi-empirical计算的对话框中出现。
Charge:指定额外的净剩电荷。额外电荷定义当前的分子系统是一个电中性系统,正电系统(阳离子),还是一个负电系统(阴离子)。
Spin Multiplicity:自旋多重度。闭壳分子的自旋多重度为1(单重态)。一个自由基,有一个未成对电子,自旋多重度为2(双重态)。有两个未成对电子的系统,自旋多重度一般为3(三重态)。然而在某些情况下,例如两个自由基,两个未成对电子也可能产生单态。
State:
描述系统中价电子状态。包括指定分子处于第一激发单态(Next lowest)或者Lowest电子态。
Lowest
给定自旋多重度的最低电子态。它不一定是基态。
Next lowest
给定自旋多重度(单重、双重、三重或四重态)的第一电子激发态。
在HyperChem 6.0中,semi-empirical方法通常需要给定多重度的最低能态(lowest)或者次最低能态(next lowest)的计算。由于偶数个电子的分子没有未成对电子,是闭壳层单态,所以只有最低三重态是有效的,而次最低三重态是无效的。例如,苯有偶数个电子,并且基态是闭壳层单重态。我们可以计算基态(最低单重态),第一激发单重态(次最低单重态),或者第一激发三重态(最低三重态)。也就是说,或者 HOMO被两个电子占据,或者一个电子在HOMO,另一个电子在LUMO,产生了激发单重态或者三重态。对双重态和四重态,只有给定多重度的最低态(lowest)可用。UHF选项仅允许给定多重度的最低态(lowest)可用。例如,可以用UHF选项研究苯的最低三重激发态,但是不能用来计算单重激发态。这是因为HyperChem中的UHF选项不允许任意的轨道占据,也不允许CI计算。对于RHF选项,可以计算CI波函数。这个计算由一系列计算得到的RHF轨道开始,或者从最低单态(或双重态),或者从half electron的单态和三态(或双态和四重态)轨道。从这些初始态可以得到参考态,从这些参考态,产生一系列微观态。通过这些微观态适当的线性组合,可以获得一系列更准确的态。

MS中自选多重度设置

Spin unrestricted: When checked, indicates that the calculation will be performed using different orbitals for different spins. This is known as a 'spin-unrestricted' or 'spin-polarized' calculation. If unchecked, the calculation uses the same orbitals for alpha and beta spins. This is known as a 'spin-restricted' or 'non-spin-polarized' calculation. Default = unchecked.

Use formal spin as initial: When checked, indicates that the initial value for the number of unpaired electrons for each atom will be taken from the formal spin introduced for each atom. This starting value will be subsequently optimized during the calculation. Default = checked.

Note. This option is enabled only if the Spin unrestricted checkbox is checked.

Multiplicity: Select the multiplicity from the dropdown list to perform a calculation on a specific spin state. Available options are:
Auto
Singlet
Doublet
Triplet
Quartet
Quintet
Sextet
Septet
Octet
When Auto is selected, DMol3 will attempt to determine the ground spin state by performing a spin-unrestricted calculation.

Note. This option is enabled only if the Spin unrestricted checkbox is checked and the Use formal spin as initial checkbox is unchecked.

Note. There is one limitation to specifying the spin state: it is not possible to force DMol3 to perform an unrestricted singlet calculation. If you check the Spin unrestricted checkbox and set the Multiplicity to Singlet, the results will be the same as if you used the Auto setting.

Note. In the case of a periodic system, the multiplicity refers to the spin state of the electrons in a single unit cell.



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