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qdbxj木虫 (小有名气)
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[求助]
三元素占有率无序精修还可以用fvar命令吗 已有1人参与
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请教一下各位老师，书上只有两种原子占有率无序的精修方法。 fvar xx xx part 1 xxxx part 2 xxxx part 0 如果我在同一位置存在三种原子的无序，还可以用fvar或者类似的方法解决吗 |
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雪国郭靖
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【答案】应助回帖
感谢参与，应助指数 +1
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用上SUMP命令就行了，下面是教程复制过来的 SUMP c sigma c1 m1 c2 m2 ... The linear restraint: c = c1*fv(m1) + c2*fv(m2) + ... is applied to the specified free variables. This enables more than two atoms to be assigned to a particular site, with the sum of site occupation factors restrained to be a constant. It also enables linear relations to be imposed between distances used on DFIX restraints, for example to restrain a group of atoms to be collinear. sigma is the effective standard deviation. By way of example, assume that a special position on a four-fold axis is occupied by a mixture of sodium, calcium, aluminium and potassium cations so that the average charge is +2 and the site is fully occupied. The necessary restraints and constraints could be set up as follows (the program will take care of the special position constraints on the coordinates and Uij of course): SUMP 1.0 0.01 1.0 2 1.0 3 1.0 4 1.0 5 ! site fully occupied SUMP 2.0 0.01 1.0 2 2.0 3 3.0 4 1.0 5 ! mean charge = +2 EXYZ Na1 Ca1 Al1 K1 ! common x, y and z EADP Na1 Ca1 Al1 K1 ! common U or Uij FVAR ... 0.20 0.30 0.35 0.15 ! starting values for free variables Na1 ... ... ... ... 20.25 ... ! 0.25*fv(2) Ca1 ... ... ... ... 30.25 ... ! 0.25*fv(3) Al1 ... ... ... ... 40.25 ... ! 0.25*fv(4) K1 ... ... ... ... 50.25 ... ! 0.25*fv(5) This particular refinement would probably still be rather unstable, but the situation could be improved considerably by adding weak SUMP restraints for the elemental analysis. Such SUMP restraints may be used when elements are distributed over several sites in minerals so that the elemental composition corresponds (within suitable standard deviations) to an experimental chemical analysis. SUMP may also be applied to EXTI and BASF parameters, including parameters used to describe twinning (TWIN). The parameters are counted in the order overall scale and free variables, BASF, thenEXTI. These virtual free variable numbers are the same as the parameter numbers printed in the .lst file for each refinement cycle. |
2楼2020-09-01 09:13:17
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