QM/MM support library

Axel Kohlmeyer, akohlmey@gmail.com Temple University, Philadelphia and ICTP, Trieste

This library provides the basic glue code to combine LAMMPS with the Quantum ESPRESSO package plane wave density functional theory code for performing QM/MM molecular dynamics simulations. More information on Quantum ESPRESSO can be found at: http://www.quantum-espresso.org

The interface code itself is designed so it can also be combined with other QM codes, however only support for Quantum ESPRESSO is currently the only option. Adding support for a different QM code will require to write a new version of the top-level wrapper code, pwqmmm.c, and also an interface layer into the QM code similar to the one in QE.

Building the QM/MM executable has to be done in multiple stages.

Step 1) Build the qmmm coupling library in this directory using one of the provided Makefile.<compiler> files or create your own, specific to your compiler and system. For example with:

make -f Makefile.gfortran

When you are done building this library, two new files should exist in this directory:

libqmmm.a the library LAMMPS will link against Makefile.lammps settings the LAMMPS Makefile will import

Makefile.lammps is created by the make command by simply copying the Makefile.lammps.empty file. Currently no additional dependencies for this library exist.

Step 2) Build a standalone LAMMPS executable as described in the LAMMPS documentation and include the USER-QMMM package. This executable is not functional for QM/MM, but it will usually be needed to run all MM calculations for equilibration and testing and also to confirm that the classical part of the code is set up correctly.

Step 3) Build a standalone pw.x executable in the Quantum ESPRESSO directory and also make the "couple" target. At the time of this writing (April 2014) you have to download a QE snapshot from the SVN repository, since no official release with the library interface to couple other codes to QE has been made yet. This is also needed to confirm that corresponding code is working correctly and to run test calculations on the cluster of QM atoms.

Step 4) To compile and link the final QM/MM executable, which combines the compiled sources from both packages, you have to return to the lib/qmmm directory and now edit the Makefile.<compiler> for the Makefile configuration used to compile LAMMPS and also update the directory and library settings for the Quantum ESPRESSO installation.

The makefile variable MPILIBS needs to be set to include all linker flags that will need to be used in addition to the various libraries from _both_ packages. Please see the provided example(s).

"make -f Makefile.<compiler> all" will now recurse through both the Quantum ESPRESSO and LAMMPS directories to compile all files that require recompilation and then link the combined QM/MM executable.

If you want to only update the local objects and the QM/MM executable, you can use "make -f Makefile.<compiler> pwqmmm.x"

Please refer to the specific LAMMPS and Quantum ESPRESSO documentation for details on how to set up compilation for each package and make sure you have a set of settings and flags that allow you to build each package successfully, so that it can run on its own.

How it works.

This directory has the source files for an interface layer and a toplevel code that combines objects/libraries from the QM code and LAMMPS to build a QM/MM executable. LAMMPS will act as the MD "driver" and will delegate computation of forces for the QM subset of the QM code, i.e. Quantum ESPRESSO currently. While the code is combined into a single executable, this executable can only act as either "QM slave", "MM slave" or "MM master" and information between those is done solely via MPI. Thus MPI is required to make it work, and both codes have to be configured to use the same MPI library.

The toplevel code provided here will split the total number of cpus into three partitions: the first for running a DFT calculation, the second for running the "master" classical MD calculation, and the third for a "slave" classical MD calculation. Each calculation will have to be run in its own subdirectory with its own specific input data and will write its output there as well. This and other settings are provided in the QM/MM input file that is mandatory argument to the QM/MM executable. The number of MM cpus is provided as the optional second argument. The MM "slave" partition is always run with only 1 cpu thus the minimum required number of MM CPU is 2, which is also the default. Therefore a QM/MM calculation with this code requires at least 3 processes.

Thus the overall calling sequence is like this:

mpirun -np <total #cpus> ./pwqmmm.x <QM/MM input> [<#cpus for MM>]

A commented example QM/MM input file is given below.

To run a QM/MM calculation, you need to set up 4 inputs, each is best placed in a separate subdirectory:

1: the total system as classical MD input. this becomes the MM master and in addition to the regular MD setup it needs to define a group, e.g. "wat" for the atoms that are treated as QM atoms and then add the QM/MM fix like this:

fix 1 wat qmmm

2: the QM system as classical MD input This system must only contain the atom (and bonds, angles, etc) for the subsystem that is supposed to be treated with the QM code. This will become the MM slave run and here the QM/MM fix needs to be applied to all atoms:

fix 1 all qmmm

3: the QM system as QM input This needs to be a cluster calculation for the QM subset, i.e. the same atoms as in the MM slave configuration. For Quantum ESPRESSO this is a regular input which in addition contains the line

tqmmm = .true.

in the &CONTROL namelist. This will make the include QE code connect to the LAMMPS code and receive updated positions while it sends QM forces back to the MM code.

4: the fourth input is the QM/MM configuration file which tells the QM/MM wrapper code where to find the other 3 inputs, where to place the corresponding output of the partitions and how many MD steps are to run with this setup.

configuration file for QMMM wrapper

mode mech # coupling choices: o(ff), m(echanical), e(lectrostatic) steps 20 # number of QM/MM (MD) steps verbose 1 # verbosity level (0=no QM/MM output during run) restart water.restart # checkpoint/restart file to write out at end

QM system config

qmdir qm-pw # directory to run QM system in qminp water.in # input file for QM code qmout NULL # output file for QM code (or NULL to print to screen)

MM master config

madir mm-master # directory to run MM master in mainp water.in # input file for MM master maout water.out # output file for MM master (or NULL to print to screen)

MM slave config

sldir mm-slave # directory to run MM slave in slinp water_single.in # input file for MM slave slout water_single.out # output file for MM slave (or NULL to print to screen)