M04A Conjugate Gradient Method in MD++

Manual 04A for MD++
Conjugate Gradient Method in MD++

Keonwook Kang and Wei Cai

Mar 19 , 2009

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Conjugate Gradient Method

The conjugate gradient (CG) method was initially proposed as an iterative method to solve a large linear system, such as $\mathrm {A} \mathbf {x} =\mathbf {b}$, where A is an n by n symmetric (positive-definite) matrix, and b is an n by 1 vector. Later, Fletcher and Reeves adopted it to solve nonlinear optimization problems. In MD++, the CG method is used to find a (local) minimum structure by minimizing a given (usu. nonlinear) potential energy function.

Algorithmically, the structure of the CG method is composed of two nested loops. The outer loop tries different search directions. The inner loop aims to find the optimal step length which minimizes the object function along a given search direction. When a certain condition is met, the program exits out of the loops and stops. The algorithm is shown in figure 1.

Figure 1: Algorithm of the CG method.

In figure 1, Ψ(x) is the potential function to be minimized, g is the gradient of Ψ at x, and s is the search direction. α_k is the optimal step length that minimizes the function Ψ(x_k+α_ks_k) at the k-th iteration. β_k is the parameter related with update of the search direction.

Depending on how we choose α_k and β_k, there can exist various different ways of the minimization method. For example, if β_k=0, it is the steepest descent method. Even in the steepest descent method, the method of finding α may not be unique for nonlinear problems. While there is the exact expression of α for a linear system, given as

$\alpha _{k}={\frac {\mathbf {g} _{k}^{T}\cdot \mathbf {g} _{k}}{\mathbf {g} _{k}^{T}A\mathbf {g} _{k}}}$,

for a nonlinear optimization problem, we need to determine α numerically, via Newton's method, quasi-Newton method, secant method, or interpolation method. These methods of finding an optimal α are called as the line search methods. For details of the line search method, refer Nocedal and Wright's Numerial Optimization

For the CG method, the search directions are constructed by conjugation of the residuals, or

$\mathbf {r} _{i}^{T}\mathbf {r} _{j}=0$      for      $i\neq j$

where r_i is the residual at i-th iteration and is defined as r_i = -g_i in nonlinear CG. From this condition, β is obtained as

$\beta \equiv$

The benefit of doing this is

Fletcher-Reeves

Polak-Ribiere

Hestenes-Stiefel

$x_{k+1}=x_{k}+\alpha _{k}s_{k}$.

where $\alpha _{k}$ and $s_{k}$ are the step length and the search direction at k-th iteration, respectively, and k is the index of the outer loop.

Comparision of the CG methods in different implementation

In the first part of this section, I compare the computation results obtained from CG relxation between MD++ (ver.Mar202009) and LAMMPS (ver.Mar262009). The test system is a 3[100] x 3[010] x 3[001] Si diamond cubic structure stretched by 10% in all three directions with two missing atoms (of atom ID 0 and 99). The number of atoms in the test system is 214. The potential fuction is MEAM Siz with the cut-off radius r_c=6(Å) The CG method is implemented by using the Polak-Ribiere method (PR+) and the backtracking method. The energy and the force tolerances are given as etol = 1e-6 (no unit) and ftol = 1e-8 (eV/Å), respectively. In the 1st column of table 1, the potential energies of the perfect structure from MD++ and LAMMPS are compared, and they are identical. In the 2nd column, the potential energies of the defect structure before relaxation are compared, and they are again same. In the 4th column, the energies of the relaxed structure are listed in different colors depending on the values of d_max (0.1, 0.01, and 0.001), and we can say that the both results are effectively identical. For this small test system, MD++ is faster than LAMMPS, because of less evaluation of the potential function. For a larger system, LAMMPS can perform better due to its ability of parallel run.

Table 1. Relaxation of 3[100]x3[010]x3[001] Si structure with a defect. Potential file : MEAM Siz w/ r_c = 6 (A). CG method = (PR+) + (backtracking). Conditions: etol = 1e-6 (no unit), ftol = 1e-8 (eV/Å). Compiler = intel compiler

	Eperf (eV)	Edefect (eV)	dmax (Å)	Erlx (eV)	Iter. No.	No. of Calls	CPU time
MD++ (PR+)	-1.00008000022397e+03	-9.00066105589930e+02	0.1	-9.04000029718829e+02	15	46	0.30002
			0.01	-9.04000167032814e+02	46	51	0.33202
			0.001	-9.03987772936424e+02	424	425	2.2201
LAMMPS (Serial Run)	-1.00008000022397e+03	-9.00066105589930e+02	0.1	-9.04000029718830e+02	15	60	0.75296
			0.01	-9.04000167032815e+02	46	96	1.2149
			0.001	-9.03987772936423e+02	424	848	10.281

The distinct difference of the CG method between MD++ and LAMMPS lies in the fact that MD++ runs the CG method in scaled coordinate (x^*) while LAMMPS does in real coordinate (x). Because of this scaling effect, d_max must be devided by the box length L for MD++, or d^*_max = d_max/L. By the same reason, the force tolerance in MD++ needs to be multiplied by the box length, or ftol^* = L·ftol.

MD++ is equipped with another implementation of the CG method, called as "zxcgr". This "zxcgr" code was written by Dongyi Liao at MIT in 1999 based on IMSL's U2CGG.F, and used the Hestenes-Stiefel method and the quadratic spline interpolation. Now I will compare two implemetations; MD++(zxcgr) and MD++(PR+). Since it is only the force tolerance that the "zxcgr" code considers to check the convergence, the energy tolerance will be set to be 0 in the comparison below. In table 2, the numbers are same upto 1e-8. Note that the energy obtained by the PR+ method becomes lower than the number (in table 1) from the same method with nonzero etol (=1e-6). This further relaxation can be understood, if we consider that in the case of nonzero energy tolerance a small step makes the energy change small enough to satisfy the energy tolerance and the structure is prone to stop relaxing further even though it is able to do because of its nonzero gradient (or force).

Table 2. Relaxation of 3[100]x3[010]x3[001] Si structure with a defect. Potential file : MEAM Siz w/ r_c = 6 (A). Conditions: etol = 0 (no unit), ftol = 1e-4 (eV/Å).

	dmax (Å)	Erlx (eV)	Iter. No.	No. of Calls	CPU time
MD++ (zxcgr)		-9.04000245866814e+02	31	45	0.31602
MD++ (PR+)	0.1	-9.04000245866095e+02	35	124	0.70004
	0.01	-9.04000245869413e+02	63	117	0.70004
	0.001	-9.04000245869795e+02	460	483	2.6002

At this point, we may raise a question. Does the scaling effect ever affect the relaxation of a structure in a non-cubic cell? Let's say that we have a 6[100] x 3[010] x 3[001] Si diamond cubic structure stretched by 20% in x direction with two missing atoms (of atom ID 0 and 99). The relaxation results are listed in table 3 below.

Table 2. Relaxation of 6[100]x3[010]x3[001] Si structure with a defect. Potential file : MEAM Siz w/ r_c = 6 (A). Conditions: etol = 0 (no unit), ftol = 1e-4 (eV/Å). For the PR+ method, d_max = 0.1 (Å). No. of atoms = 430

	Eperf (eV)	Edefect (eV)	Erlx (eV)	Iter. No.	No. of Calls
MD++ (zxcgr)	-2.00016000044795e+03	-1.86579478735442e+03	-1.86764046397026e+03	129	175
MD++ (PR+)			-1.86764046392565e+03	168	686
LAMMPS	-2.00016000044795e+03	-1.86579478735442e+03	-1.86764046393067e+03	74	387

The relaxed energy values are same upto 1e-6.