- Energy minimization
**Energy minimization**(energy optimization) methods are common techniques to compute the equilibrium configuration ofmolecules . The basic idea is that a stable state of a molecular system should correspond to a local minimum of theirpotential energy . This kind of calculation generally starts from an arbitrary state of molecules, then the mathematical procedure ofoptimization allows us to move atoms ("to vary variables") in a way to reduce the net forces (thegradients ofpotential energy ) to nearly zero. Likemolecular dynamics and Monte-Carlo approaches,periodic boundary conditions have been allowed in energy minimization methods, to make small systems. A well established algorithm of energy minimization can be an efficient tool for molecular structure optimization.Unlike

molecular dynamics simulations, which are based on Newtonian dynamic laws and allow calculating atomic trajectory withkinetic energy , molecular energy minimization does**not**include the effect of temperature, and hence the trajectories of atoms during the calculation do not really make any physical sense, i.e. we can only obtain a final state of system that corresponds to a local minimum of potential energy. From physical point of view, this final state of the system corresponds to the configuration of atoms when the temperature of system infinitely approximate to zero, e.g. as shown in Figure 1, if there is a cantilevered beam vibrating between positions 1 and 2 around an equilibrium position 0 with an initial kinetic motion, no matter we start with the state 1, the state 2 or any other state between these two positions, the result of energy minimization for this system will always be the state 0.The

algorithms of gradient are the most popular methods for energy minimization. The basic idea of gradient methods is to move atoms by the total net forces acting on them. The force on atoms is calculated as the negative gradient of total potential energy of system, as follows::$extstyle\; Fleft(\; r\_\{i\}\; ight)\; =-overrightarrow\{\; abla\}\_\{r\_\{iU^\; ext\{tot\},quad\; i=1,ldots,N,$

where

**r**_{"i"}is the position of atom "i" and "U"^{tot}is the total potential energy of the system.An analytical formula of the gradient of potential energy is preferentially required by the gradient methods. If not, one needs to calculate numerically the

derivative s of the energy function. In this case, thePowell's direction set method or thedownhill simplex method can generally be more efficient than the gradient methods.**Simple gradient method (**steepest descent ,gradient descent )Here we have a single function of the potential energy to minimize with 3N independent

variables , which are the 3 components of thecoordinates of N atoms in our system. We calculate the net force on each atom**F**at eachiteration step "t", and we move the atoms in the direction of**F**with a multiple factor "k". "k" can be smaller at the beginning of calculation if we begin with a very highpotential energy . Note that similar strategy can be used inmolecular dynamics for reducing the probability ofdivergence problems at the beginning of simulations.:$r\_\{i\}^\{t\}=r\_\{i\}^\{t-1\}+kappacdot\; Fleft(\; r\_\{i\}\; ight)\; ,quad\; i=1,ldots,N.$

We repeat this step in the above equation "t" = 1,2,... until

**F**reaches to zero for every atom. The potential energy of system goes down in a long narrow valley of energy in this procedure.Despite that it is as well called “steepest descent”, the simple gradient

algorithm is in fact very time-consuming if we compare it to theconjugate gradient approach, it is therefore known as a "not very good" algorithm. However, its advantage is itsnumerical stability , i.e., the potential energy can never increase if we take a reasonable "k". Thus, it can be combined with a conjugated gradient algorithm for solving the numerical divergence problem when two atoms are too close to each other.Conjugate gradient method The conjugate gradient algorithm includes two basic steps: adding an

orthogonal vector to the direction of research, and then move them in another direction nearly perpendicular to this vector. These two steps are as well known as: "step on the valley floor and then jump down". Figure 2 shows a highly simplified comparison between the conjugated and the simple gradient on a 1D energy curve.In this algorithm, we minimize the energy function by moving the atoms as follows,

:$r\_\{i\}^\{t\}=r\_\{i\}^\{t-1\}+kappacdot\; h\_\{i\}^\{t\},quad\; i=1,ldots,N,$

where

:$h\_\{i\}^\{t\}=Fleft(\; r\_\{i\}^\{t\}\; ight)\; +gamma\_\{i\}^\{t-1\}h\_\{i\}^\{t-1\}$

and "gamma" is updated using the

Fletcher-Reeves formula as::$gamma\_\{i\}^\{t-1\}=frac\{Fleft(\; r\_\{i\}^\{t\}\; ight)\; cdot\; Fleft(\; r\_\{i\}^\{t\}\; ight)\; \}\{Fleft(\; r\_\{i\}^\{t-1\}\; ight)\; cdot\; Fleft(\; r\_\{i\}^\{t-1\}\; ight)\; \}$

Here we note that "gamma" can also be calculated by using the Polak-Ribiere formula, however, it is less efficient than the Fletcher-Reeves one for certain energy functions. At the beginning of calculation (when "t" = 1), we can make the search direction vector

**h**0 = 0.This algorithm is very efficient. However, it is not quiet stable with certain potential functions, i.e. it sometimes can step so far into a very strong repulsive energy range (e.g. when two atom are too close to each other), where the

gradient on this point is almost infinite. It can directly result a typical data-overrun error during the calculation. For resolving this problem, we can combine the conjugated gradient algorithm with the simple one. Figure 3 shows the schematics of this combined predicting algorithm. We note for implementation that the steps 2 and 5 can be combined to one single step.**Boundary conditions**The atoms in our system can have different degrees of freedom. For example, in case of a tube suspended over two supports, we need to fix certain number of atoms "N*" at the tube ends during the calculation. In this case, it is enough not to move these "N*" atoms in the step 4 or 8 in Figure 3, but we still calculate their interaction with other atoms in the steps 2 and 5. i.e. from mathematical point of view, we change the total number of variables in the energy function from "3N" to "3N-3N*"using the boundary condition, by which the values of these "3N*" unknown variables are taken as known constants. Note that one can even fix atoms in only one or two directions in this way.

Moreover, one can equally adding other boundary conditions to the minimized energy function, such as adding external forces or external electric fields to the system. In these cases, the terms in potential energy function will be changed but the number of variables remains constant.

Here an example of the application of the energy minimization method in molecular modeling in

nanoscience is shown in Figure 4..]

Further information about the application of this method in

nanoscience and Computational Codes programmed inFortran for students is available in the following external links.**External links*** [

*http://www.nrbook.com/a/bookfpdf.php Numerical Recipes in Fortran 77*]**Additional references*** Payne et al , "Iterative minimization techniques for ab initio total-energy calculations: Molecular dynamics and conjugate gradients", "Reviews of Modern Physics" 64 (4), pp. 1045–1097. (1992)

* Atich et al, "Conjugate gradient minimization of the energy functional: A new method for electronic structure calculation", "Physical Review B" 39 (8), pp. 4997–5004, (1989)

* Chadi, "Energy-minimization approach to the atomic geometry of semiconductor surfaces", "Physical Review Letters" 41 (15), pp. 1062–1065 (1978)

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