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【求助】有人能解释一下局部优化吗？
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| 局部优化在高斯中有几个固定的关键字，但是有哪位高手给指点一下具体应该怎么办？ |
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虚谦
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2楼2008-09-28 12:08:43
3楼2008-09-28 16:44:31
jysgg1015
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4楼2008-09-29 21:36:56
abbott
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gwdavid(金币+2,VIP+0):谢谢交流！欢迎再次光临模拟版！
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我是一知半解，我知道的东西，贴在下面，希望对你有用。 1 关于优化的基本概念 Geometry Optimization - Basic Considerations 1。 How much output is generated during geometry optimization (and actually all other types of Gaussian calculations as well) depends on the beginning of the keyword line: #P will provide somewhat more detailed output # will provide normal output #T will provide somewhat less output 2。 Geometry optimization can be performed in three different types of coordinate systems. Please observe that the way, the geometry is defined in the input file, is actually separate from deciding, in which coordinate system the optimization will be performed: opt=Z-Matrix will optimize the geometry in internal coordinates (as provided in the input file) opt or opt=Redundant will optimize the geometry in redundant internal coordinates (chosen automatically) opt=Cartesian will optimize the geometry in Cartesian coordinates For all three options, the optimization algorithm will vary the structure of the system until changes in the gradient and the structure on two successive iterations are smaller than prefixed values (convergence criteria). For each step of the geometry optimization, Gaussian will write to the output file a) the current structure of the system, b) the energy for this structure, c) the derivative of the energy with respect to the geometric variables (the gradients), d) a summary of the convergence criteria. 3。 For all methods used in Gaussian, the energy will be given in atomic units (au). The atomic unit of energy is called Hartree and equates to other energy units as follows: 1 Hartree = 627.15 kcal/mol 1 Hartree = 2625.5 kJ/mol 1 Hartree = 27.2116 eV 1 Hartree = 4.3597482*10-18 J/particle The energies of molecular systems as calculated by most quantum mechanical methods called "ab initio" are calculated relative to separate electrons and nuclei. Energies for molecular systems are therefore very large and negative. In the output files the energies are prefaced by "SCF" and the UNIX command grep "SCF" output.file can therefore be used to list all energies contained in a Gaussian output file. Some of the theoretical methods available in Gaussian such as AM1, MNDO, or PM3 produce heats of formation (that is, the energy of the system relative to its constituent elements in their standard states at 298.15K and 1 atm(101325 Pa). Heats of formation can be either positive or negative and are comparatively small. By definition the heats of formation of the elements are zero. The heats of formation contained in the output file can be listed using grep "Energy= " output.file 4。 After each iteration of the geometry optimization, the output files contain a summary of the current stage of the optimization: Item value Threshold Converged? Maximum Force 0.021672 0.000450 NO RMS Force 0.018596 0.000300 NO Maximum Displacement 0.038954 0.001800 NO RMS Displacement 0.033876 0.001200 NO Predicted change in Energy=-1.250480D-03 The first two lines contain the maximum remaining force on an atom in the system as well as the average (RMS, root mean square) force on all atoms. In any case of doubt, this information is given in atomic units (here: Hartrees/Bohr and Hartrees/Radians). Together with the actual value for the current structure appears the Threshold value. The third and fourth convergence criteria are the maximum displacement, that is, the maximum structural change of one coordinate as well as the average (RMS) change over all structural parameters in the last two iterations. Once the current values of all four criteria fall below the threshold, the optimization is complete. The convergence criteria can be changed in two different ways: 1) Using the opt keyword opt without any additional information sets the RMS force criterion to 3*10-4 opt=tight will set the RMS force criterion to 1*10-5 and scale the other three criteria accordingly opt=verytight will set the RMS force criterion to 1*10-6 and scale the other three criteria accordingly 2) Using the IOP keyword (Internal Option) iop(1/7=x) will set the RMS force criterion to x*10-6 and scale the other three criteria accordingly. Thus, using iop(1/7=10) one can obtain the same final result as with opt=tight. Choosing tighter convergence criteria will, of course, give improved results but will also need more computer time. The default settings are appropriate for small systems. Especially for large structures, however, convergence of the last two criteria can be very slow and it is sometimes advisable to stop optimizations before all four criteria are fulfilled. The maximum number of optimization cycles depends on the size of the system and is automatically adjusted by Gaussian. If a particular setting of optimization cycles is desired, however, this can be specified using opt=(maxcycles=n) 5。 The default optimization algorithm included in Gaussian is the "Berny algorithm" developed by Bernhard Schlegel. This algorithm uses the forces acting on the atoms of a given structure together with the second derivative matrix (called the Hessian matrix) to predict energetically more favorable structures and thus optimize the molecular structure towards the next local minimum on the potential energy surface. As explicit calculation of the second derivative matrix is quite costly, the Berny algorithm constructs an approximate Hessian at the beginning of the optimization procedure through application of a simple valence force field, and then uses the energies and first derivatives calculated along the optimization pathway to update this approximate Hessian matrix. The success of the optimization procedure therefore depends to some degree on how well the approximate Hessian represents the true situation at a given point. For many "normal" systems, the approximate Hessians work quite well, but in a few cases a better Hessian has to be used. Often it is sufficient to calculate the Hessian matrix explicitly once at the beginning of the calculation and then use the standard updating scheme of the Berny algorithm. This is specified using the opt=calcfc keyword. In some very rare cases, the Hessian changes considerably between optimization steps and must then be recomputed after each optimization step using the opt=calcall keyword. In case a number of different options are to be specified for geometry optimization, these options must be given in parenthesis: opt=(Z-Matrix,calcfc,tight,maxcycles=25) [ Last edited by abbott on 2008-9-29 at 22:05 ] |
5楼2008-09-29 21:56:00
abbott
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gwdavid(金币+2,VIP+0):谢谢交流！欢迎再次光临模拟版！
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为什么进行局部优化： 冻结坐标有时是有用的，举几个例子： 1，如优化溶质分子和nH2O的supermolecule，周围的水分子，可能结构变化不大，这时就可以固定水的键长或键角，减少工作量。 2，如用cluster模拟大块固体，这时也许就要按照晶格参数固定原子键长和键角。 3，又如不固定NH3－H－H2O中N－O键长是得不到质子传递势垒。 4，还有在优化稳定构型或过渡态得时候也有时用到部分优化得到接近得结果，再放开优化，这样逐步得到你想要得结果。 关于部分优化的实例，我目前就是采用下面的方法做的。 opt=z-matrix的部分优化。 %Chk=reac-hf3 #p RHF/STO-3G opt(z-matrix,maxcyc=500) freq=noraman scf(maxcyc=200) reactant fopt 1 1 N -1 -1.75389 -0.48105 0.31424 H 0 x2 y2 z2 H 0 x3 y3 z3 H 0 x4 y4 z4 O -1 2.16296 0.18089 -0.15503 H 0 x6 y6 z6 H 0 x7 y7 z7 H 0 x8 y8 z8 x2=-2.2088 y2=0.39705 z2=0.46255 x3=-2.14589 y3=-0.9279 z3=-0.4899 x4=-1.88451 y4=-1.06386 z4=1.11629 x6=1.36256 y6=0.28882 z6=-0.67401 x7=2.32973 y7=0.98241 z7=0.34634 x8=2.90378 y8=0.01009 z8=-0.74115 *********************************************** opt=modredundant的部分优化。 分子坐标部分可以为具体的数值，而不用为参数形式。 %Chk=reac-hf3 #p RHF/STO-3G opt(modredundant,maxcyc=500) freq=noraman scf(maxcyc=200) reactant fopt 1 1 N -1 -1.75389 -0.48105 0.31424 H 0 H 0 H 0 O -1 2.16296 0.18089 -0.15503 H 0 H 0 H 0 1 5 F 希望这些信息对你有用。 以上的东西转自厦大论坛 [ Last edited by abbott on 2008-9-29 at 22:08 ] |
6楼2008-09-29 22:07:17
7楼2008-12-01 10:34:12
xuefei06
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8楼2008-12-03 14:17:37