So, I’ve been using Quantum ESPRESSO (QE) for a while now. And it’s relaxation (optimization) functionality as well. Now, although I had a basic understanding of how all the other calculations are done in QE, I didn’t quite know anything about the optimization strategies. Of course I had some basic ideas but nothing concrete. So I decided to spend some time in learning some of the basics of geometry optimization and implementing those using QE.

In this post, currently, I have implemented Conjugate Gradient using Polak-Riberie’s formula as well as Steepest Descent method. Let me talk about the two methods briefly.

### Steepest Descent (Gradient Descent)

This is the most simple technique to get the local minima or maxima of a function (in our case energy). One, simply, calculates the gradient (which is the direction of the steepest increase) and move in the negative direction of the gradient (which is simply the force in our case) to get to the minima which is what we desire. Then the value of the function (energy) is calculated at the new position and checked if it meets the convergence requirement or not, i.e. is the gradient 0 or not.

The coordinates our updated according to the equation:

where is the step-size or step-length. The step-length should be chosen such that it minimizes such that and are fixed. However, in reality, such an exact minimization is rarely performed. Rather, the value of that gives a sufficient decrease in is enough.

### Conjugate Gradient

Do line minimizations in a direction which is a combination of the current gradient and the previous one

The coordinates our updated according to the equation:

where

The different conjugate-gradient methods provide different ways to choose

; they involve dot products of current and previous gradients, e.g., Polak-Ribiere:

Now as you must have noticed by now, you would need the gradient of energy, i.e. the forces, to make the above algorithms work for geometry optimization using QE. Now QE can calculate and give you the forces as well as the total energy, so we have a way to access all the necessary information we are going to need.

A few questions that can come to mind regarding the above algorithms are answered below:

#### Using the gradient vector or the gradient unit vector?

Now in most of the literature, people generally use gradient vector but I also found some literature using gradient unit vectors for the descent direction. So I ran some calculations using that but the results didn’t offer anything conclusive. It didn’t seem to make any much of a difference. The code and results are shown below. I guess I would stick with using the gradient vector rather than unit vectors.

#### Constant step-length or adaptive step-length

Now ideally the step-length should be determined at each iteration, by optimizing this function. That is the value of that gives the most decrease in the objective function, which is energy for our case. This determination of is done by a one-dimensional optimization. But the thing is that, this means you are doing an optimization inside an optimization and determining a exactly would lead to a lot of wastage of time and cpu resources. So alternatively, you could just use an inexact line search to find that gives sufficient decrease or satisfies either one of the following conditions: Armijio, Wolfe, Strong Wolfe, Goldstein, etc.

However, even in inexact line search you would have to calculate the total energy and gradients and this leads to a increased computation time. In a popular method known as backtracking line search you start with a large and then see if it satisfies the sufficient decrease condition or not. If it does then you can take a large step in that direction. If not then you divide by 2 or some integer and calculate the energy again and repeat the procedure until you find the appropriate step-length to move in the descent direction.

But in my experience, if you start out with a good approximation to the starting geometry then the advantage provided by the starting large step-length is nullified by the need of making it smaller once you’re near the minimum. That is after taking the first large step, let’s say you’re now in the vicinity of the minimum. Now if you take the same large step-length then you would overshoot the minimum and energy will increase. So you would iteratively make smaller until the energy decreases. But if you had to make considerably smaller at this step, then for the remaining iterations you would be stuck with a very small value of step-length that would take a long time to converge to the minimum. Alternatively you could reset the step-length to a larger value for the next iteration, so that at each optimization step you’re taking as large a step as possible. But overall this would lead to a lot of unused energy and gradient calculations, and in some cases this could even lead to a longer time than the calculations performed with constant step-length. However, a constant step-length should be used very sensibly. It can’t be too large or too small. If it’s too large you would never converge. If it’s too small, you would take a lot of time to converge when near a minimum. An adaptive step-size would take care of these problems on it’s own. In my experience I found that a carefully chosen constant step-size could outperform an algorithm with an adaptive step-length.

If you decide to run some calculations with constant step-length, here’s what I would suggest on how to choose it. First run a simple total energy calculation on your initial system and calculate the forces. This would give you an idea of how good is your starting approximation. If the forces are very low then this means you are close to the final (optimized) geometry. Based on this information you can decide on how big a step-length you’d need. For example, for the case of H2 molecule I found that using constant gave the optimized geometry in 5-7 scf calculations using both steepest descent as well as conjugate gradient method. However using adaptive step-size and starting with took at least 16 scf calculations because of the need to make smaller at some iterations. However, I suppose that using adaptive step size with initial might have resulted in an even faster convergence. [Will add the results if I do this]

So these were the merits and demerits of both the constant or adaptive step length.

#### Parallelization:

Another idea that I have, is to parallelize the process of choosing the . Say you start out with initial . Now if you have 4 processors available, then you can run 4 separate instances of scf total energy calculations using different . Since all of these calculations should take approximately the same time, so at the end you would have the necessary information to decide which is the biggest value of that satisfies the sufficient decrease condition. This method although leads to a lot of unused and unnecessary calculations if \alpha=1 already satisfied the sufficient decrease condition, but the time taken would be less than the methods discussed previously. Let’s say we started with but that was too large, and is the value that will satisfy the sufficient decrease condition. In this case if we iteratively decrease then in a serial algorithm we would end up with 4 scf cycles if we divide by 2 at each iteration. However, in the parallel algorithm we would still perform the 4 scf cycles but they would be performed all at once leading to only one-fourth of computational time. I might implement this soon. But first I need to learn more about parallelization as I am not very familiar with that. Also, if the calculations are small enough I might also try running just 4 instances with different \alpha on the same processor, but this would be beneficial only as long as a single scf calculation isn’t computationally expensive enough to take up all the available power of the single processor. If the calculations are simple and small and don’t take up a lot of resources on the single processor then one could just run multiple instances with different .

#### Comparing the results with the BFGS algorithm as implemented in Quantum ESPRESSO

Unfortunately, none of the Conjugate gradient or Steepest Descent variations mentioned here could come even close to BFGS algorithm as implemented in Quantum ESPRESSO. For small systems such as H2, N2 or O2, the results were comparable. But for systems with more degrees of freedom such as ZnS, CdS, Zn3S3, the convergence towards the minima took ages as compared to BFGS. The results are given below. I mean of course I expected it (CG and SD) to be slow but not this slow. So this was rather frustrating as I spent like a week working on this: writing codes for different variations, troubleshooting and running test calculations. I understand that there could be some problems with my implementations of Conjugate Gradient and Steepest Descent too. But this seems rather unlikely. However, if you’re reading this and have some suggestions or find some faults, please do let me know. Anyway my next goal is to implement the BFGS algorithm and see how does that fare in comparison to the one implemented by QE. QE uses trust region based BFGS, while I’m planning on first implementing it with an inexact line search using the wolfe conditions and bracketing and zooming the suitable .

#### Using Wolfe conditions or not?

So as I mentioned before the suitable step-length should satisfy some set of conditions. However, I read that if you are using backtracking line search as the method for inexact line search for suitable then there is no need to check for the second wolfe condition that makes sure that the steps aren’t too small. This is because we are dividing by 2 or some integer, and this makes sure that we are within a striking distance within the previous larger value, so obviously this can’t be too small. Although, I still did write a code that checked for the decrease in the function as well as the wolfe 2 condition, but that gave strange results. So probably my implementation was faulty, but I also think that maybe that’s why it isn’t suggested for backtrack line search. Also, I didn’t use the sufficient decrease condition (Armijio or Wolfe 1) in any of the implementations mentioned here. As it was complicated to implement. Rather I just checked that the newer energy was less than the last energy, i.e. ensure that we were moving in a descent direction.

So I’ve spent quite some time discussing the various aspects of implementing a geometry optimization scheme using QE or any other DFT package. There are still lot’s of stuff that I didn’t cover, like using the cartesian coordinates or internal coordinates, etc. Anyway, the following are the various codes (shell scripts if you will) that I used for implementing various schemes. The scripts should be easily runnable on any Linux. You would have to just set the correct `pw.x`

path and pseudopotential path. I’ve tried to provide useful comments in the code. Hope that helps.

### Steepest Descent (with inexact line search for 1d optimization of step length)

The following is a shell script to implement Steepest Descent using backtracking line-search. In this code the value of is set to be 1 but is decreased by half if the value of new energy is not less than the energy in last step. This code also resets the value of to go back to 1 once sufficient decrease is satisfied. I also share another version of this code where once the value of is decreased, the rest of the iterations are also performed with that value, and the value is not reset. One demerit of that is that if the has to be made very small for some iteration, then for the remaining iterations also we are stuck with this small step-length. But this kind of behaviour could be advantageous if one is very near the minimum.

#### run.sh:

#!/bin/bash #Script to find the optimized coordinates of a molecule/cluster using the Steepest Descent (Gradient Descent) algorithm interfaced with Quantum ESPRESSO tol=$1 a=1 echo "A" > a.txt chmod u+x scfCreator.sh #-------------------------------- #Algorithm #--------------------------------- #1. Run an SCF calculation using the user given starting geometry (atomic coordinates and PP info, etc.) #2. Extract the information of forces acting on each of the atoms as that would give us the gradient of energy for each atom. #3. Update the atomic coordinates for the nth atom using: # x{i,n}_new=x{i,n}_old + aF{i,n} # where i is for the ith coordinate and n is for nth atom and F is the force, i.e. negative of gradient, a is somescaling factor #4. Keep repeating until the forces are minimized #Find out no. of atoms nat=$(awk '{for (I=1;I<=NF;I++) if ($I == "nat") {print $(I+2)};}' geom.in) #Create an SCF file with the initial coordinates iter=1 grep -A $nat 'ATOMIC_P' geom.in >coordIter$iter.txt flag=1 while [ $flag == 1 ] do flag=0 iter1=$(($iter+1)) ./scfCreator.sh 55 6 6 6 38 Iter$iter coordIter$iter.txt mpirun '/mnt/oss/dmishra_du/QE/qe-6.1/bin/pw.x' -inp scfIter$iter.in> scfIter$iter.out echo "Forces" >forcesIter$iter.txt #echo "ATOMIC_POSITIONS {angstrom}" >coordIter$iter1.txt if [ $iter != 1 ] then E1=$(awk '{for (I=1;I<=NF;I++) if ($I == "!") {print $(I+4)};}' scfIter$iter.out) E0=$(awk '{for (I=1;I<=NF;I++) if ($I == "!") {print $(I+4)};}' scfIter$(($iter-1)).out) if (( $(bc <<< "$E1 > $E0") )) then a=$( echo "$a/2"|bc -l) iter=$(($iter-1)) else a=1 fi #a=$( echo "$a/(($Fn1-($Fn01))*($Fn1-($Fn01))+($Fn2-($Fn02))*($Fn2-($Fn02))+($Fn3-($Fn03))*($Fn3-($Fn03)))"|bc -l) fi echo "ATOMIC_POSITIONS {angstrom}" >coordIter$(($iter+1)).txt #Run a loop for no. of atoms for (( n=1; n<=$nat; n++ )) do species=$(grep -A $nat 'ATOMIC_PO' coordIter$iter.txt | sed '1d' | awk "NR==$n{print}" |awk '{print $1}' ) #Atomic positions at the start of this iteration x1=$(grep -A $nat 'ATOMIC_PO' coordIter$iter.txt | sed '1d' | awk "NR==$n{print}" |awk '{print $2}' ) x2=$(grep -A $nat 'ATOMIC_PO' coordIter$iter.txt | sed '1d' | awk "NR==$n{print}" |awk '{print $3}' ) x3=$(grep -A $nat 'ATOMIC_PO' coordIter$iter.txt | sed '1d' | awk "NR==$n{print}" |awk '{print $4}' ) #Extract the ith component of force of the nth atom Fn1=$(grep "atom $n" scfIter$iter.out | awk '{for (I=1;I<=NF;I++) if ($I == "atom") {print $(I+6)};}') Fn2=$(grep "atom $n" scfIter$iter.out | awk '{for (I=1;I<=NF;I++) if ($I == "atom") {print $(I+7)};}') Fn3=$(grep "atom $n" scfIter$iter.out | awk '{for (I=1;I<=NF;I++) if ($I == "atom") {print $(I+8)};}') echo "$species $Fn1 $Fn2 $Fn3" >>forcesIter$iter.txt #If the force is less than the given threshold then don't update coordinate #else update the coordinates using the formula in step 3. and alert the flag if (( $(bc <<< "${Fn1#-} >= $tol") )) then x1=$( echo "$x1+$a*$Fn1"|bc) flag=1 fi if (( $(bc <<< "${Fn2#-} >= $tol") )) then x2=$( echo "$x2+$a*$Fn2"|bc) flag=1 fi if (( $(bc <<< "${Fn3#-} >= $tol") )) then x3=$( echo "$x3+$a*$Fn3"|bc) flag=1 fi echo $a >> a.txt if [ $n == $nat ] then echo "__________________" >>a.txt fi echo "$species $x1 $x2 $x3" >>coordIter$(($iter+1)).txt done #If flag is not raised the geometry is optimized geometry else go bac to SCF calculation with updated coordinates iter=$(($iter+1)) echo $flag >>flag.txt done

### Conjugate Gradient (with adaptive step length)

The following shell script implements the Conjugate Gradient algorithm using backtracking line search by interfacing with Quantum Espresso.

#### run.sh

#!/bin/bash #Script to find the optimized coordinates of a molecule/cluster using the Conjugate Gradient algorithm interfaced with Quantum ESPRESSO #Tolerance entered by the user tol=$1 #Step-length \alpha a=1 #Value of step-length at each iteration echo "A" > a.txt #Compile the input file creating script chmod u+x scfCreator.sh #-------------------------------- #Algorithm #--------------------------------- #1. Run an SCF calculation using the user given starting geometry (atomic coordinates and PP info, etc.) #2. Extract the information of forces acting on each of the atoms as that would give us the gradient of energy for each atom. #3. Update the atomic coordinates for the nth atom using: # x{i,n}_new=x{i,n}_old + aF{i,n} # where i is for the ith coordinate and n is for nth atom and F is the force, i.e. negative of gradient, a is somescaling factor #4. Keep repeating until the forces are minimized #Find out no. of atoms nat=$(awk '{for (I=1;I<=NF;I++) if ($I == "nat") {print $(I+2)};}' geom.in) #Create an SCF file with the initial coordinates iter=1 grep -A $nat 'ATOMIC_P' geom.in >coordIter$iter.txt flag=1 while [ $flag == 1 ] do flag=0 iter1=$(($iter+1)) ./scfCreator.sh 50 6 6 6 24 Iter$iter coordIter$iter.txt '/home/manas/qe-6.1/bin/pw.x' <scfIter$iter.in> scfIter$iter.out echo "Forces" >forcesIter$iter.txt if [ $iter != 1 ] then E1=$(awk '{for (I=1;I<=NF;I++) if ($I == "!") {print $(I+4)};}' scfIter$iter.out) E0=$(awk '{for (I=1;I<=NF;I++) if ($I == "!") {print $(I+4)};}' scfIter$(($iter-1)).out) if (( $(bc <<< "$E1 > $E0") )) then a=$( echo "$a/2"|bc -l) iter=$(($iter-1)) fi #a=$( echo "$a/(($Fn1-($Fn01))*($Fn1-($Fn01))+($Fn2-($Fn02))*($Fn2-($Fn02))+($Fn3-($Fn03))*($Fn3-($Fn03)))"|bc -l) fi echo "ATOMIC_POSITIONS {angstrom}" >coordIter$(($iter+1)).txt #Run a loop for no. of atoms for (( n=1; n<=$nat; n++ )) do species=$(grep -A $nat 'ATOMIC_PO' coordIter$iter.txt | sed '1d' | awk "NR==$n{print}" |awk '{print $1}' ) #Atomic positions at the start of this iteration x1=$(grep -A $nat 'ATOMIC_PO' coordIter$iter.txt | sed '1d' | awk "NR==$n{print}" |awk '{print $2}' ) x2=$(grep -A $nat 'ATOMIC_PO' coordIter$iter.txt | sed '1d' | awk "NR==$n{print}" |awk '{print $3}' ) x3=$(grep -A $nat 'ATOMIC_PO' coordIter$iter.txt | sed '1d' | awk "NR==$n{print}" |awk '{print $4}' ) #Extract the ith component of force of the nth atom Fn1=$(grep "atom $n" scfIter$iter.out | awk '{for (I=1;I<=NF;I++) if ($I == "atom") {print $(I+6)};}') Fn2=$(grep "atom $n" scfIter$iter.out | awk '{for (I=1;I<=NF;I++) if ($I == "atom") {print $(I+7)};}') Fn3=$(grep "atom $n" scfIter$iter.out | awk '{for (I=1;I<=NF;I++) if ($I == "atom") {print $(I+8)};}') echo "$species $Fn1 $Fn2 $Fn3" >>forcesIter$iter.txt if [ $iter == 1 ] then d1=$Fn1 d2=$Fn2 d3=$Fn3 else Fn01=$(grep "atom $n" scfIter$(($iter-1)).out | awk '{for (I=1;I<=NF;I++) if ($I == "atom") {print $(I+6)};}') Fn02=$(grep "atom $n" scfIter$(($iter-1)).out | awk '{for (I=1;I<=NF;I++) if ($I == "atom") {print $(I+7)};}') Fn03=$(grep "atom $n" scfIter$(($iter-1)).out | awk '{for (I=1;I<=NF;I++) if ($I == "atom") {print $(I+8)};}') B1=$(echo "$Fn1*($Fn1-($Fn01))+$Fn2*($Fn2-($Fn02))+$Fn3*($Fn3-($Fn03))"|bc) B1=$(echo "$B1/(($Fn01*$Fn01)+($Fn02*$Fn02)+($Fn03*$Fn03))"|bc -l) d1=$(echo "$Fn1+$B1*$d1"|bc) d2=$(echo "$Fn2+$B1*$d2"|bc) d3=$(echo "$Fn3+$B1*$d3"|bc) fi #If the force is less than the given threshold then don't update coordinate #else update the coordinates using the formula in step 3. and alert the flag if (( $(bc <<< "${Fn1#-} >= $tol") )) then x1=$( echo "$x1+$a*$d1"|bc) flag=1 fi if (( $(bc <<< "${Fn2#-} >= $tol") )) then x2=$( echo "$x2+$a*$d2"|bc) flag=1 fi if (( $(bc <<< "${Fn3#-} >= $tol") )) then x3=$( echo "$x3+$a*$d3"|bc) flag=1 fi echo $a >> a.txt if [ $n == $nat ] then echo "__________________" >>a.txt fi echo "$species $x1 $x2 $x3" >>coordIter$(($iter+1)).txt done #If flag is not raised the geometry is optimized geometry else go bac to SCF calculation with updated coordinates iter=$(($iter+1)) echo $flag >>flag.txt done

### scfCreator.sh

The following script creates an scf input file for QE. It takes various info (Ecut, kx, ky, kz, nbnds and filename) as parameters. To make it work on your system just modify the `pseudo_dir`

attribute giving the path to the pseudopotential files.

#!/bin/bash #SCF input file creator for a given value of Ecut and kx,ky,kz, nbnd and filename #Values of Ecut pwcut=$1 pwrho=$(($pwcut*10)) #Values of k points kx=$2 ky=$3 kz=$4 #no. of bands nb=$5 #filename suffix=$6 file=scf$suffix.in coordFile=$7 echo '&CONTROL calculation = "scf" max_seconds = 8.64000e+08 pseudo_dir = "/home/manas/Scripts/GGA_SemiConductor/pseudopot" outdir = "temp" tprnfor = .TRUE. tstress = .TRUE. /' >$file echo '&SYSTEM' >>$file #Find out the lattice information from the geom.in file and append that to input file (Using sed non-inclusive) sed -n '/&SYSTEM/,/\//p' geom.in | sed '1d;$d' >> $file echo ' degauss = 1.00000e-02 ecutrho = '$pwrho' ecutwfc = '$pwcut' occupations = "fixed" smearing = "gaussian" nbnd = '$nb' /' >>$file echo ' &ELECTRONS conv_thr = 1.00000e-06 electron_maxstep = 200 mixing_beta = 7.00000e-01 startingpot = "atomic" startingwfc = "atomic+random" / K_POINTS {gamma} '$kx' '$ky' '$kz' 0 0 0 '>> $file echo 'ATOMIC_SPECIES' >>$file sed -n '/ATOMIC_S/,/ATOMIC_P/p' geom.in | sed '1d;$d' >> $file sed -n '/ATOMIC_P/,//p' $coordFile >> $file

### References:

http://www.quantum-espresso.org/

https://en.wikipedia.org/wiki/Line_search

https://en.wikipedia.org/wiki/Gradient_descent

https://en.wikipedia.org/wiki/Conjugate_gradient_method

https://en.wikipedia.org/wiki/Wolfe_conditions

https://en.wikipedia.org/wiki/Backtracking_line_search

https://en.wikipedia.org/wiki/Quasi-Newton_method

https://en.wikipedia.org/wiki/Broyden–Fletcher–Goldfarb–Shanno_algorithm