PLUMED is a library for enhanced-sampling methods (metadynamics, umbrella sampling, ...). During an ABACUS molecular-dynamics run it can compute collective variables and apply biasing potentials, controlled by the plumed and plumed_file INPUT keywords.

Install PLUMED 2.x first, for example:

./configure --prefix=/path/to/plumed && make -j && make install

Then configure ABACUS with -DENABLE_PLUMED=ON:

cmake -B build -DENABLE_PLUMED=ON ...
cmake --build build -j

The PLUMED installation is located with pkg-config (through the plumed.pc file shipped by PLUMED); if PLUMED is installed in a non-standard prefix, add -DPLUMED_ROOT=/path/to/plumed so that it can be found without pkg-config. When ABACUS is built without -DENABLE_PLUMED=ON, setting plumed in INPUT stops the run with an explicit error message.

In the INPUT file:

plumed          1            # enable the PLUMED interface
plumed_file     plumed.dat   # PLUMED input file (default: plumed.dat)

The PLUMED input file defines the collective variables and the biasing potential, e.g. for a dihedral angle:

UNITS LENGTH=A TIME=fs
t: TORSION ATOMS=3,1,2,7
metad: METAD ARG=t PACE=50 HEIGHT=2.0 SIGMA=0.15 FILE=HILLS BIASFACTOR=20 TEMP=600
PRINT ARG=t,metad.bias FILE=COLVAR STRIDE=1

ABACUS hands the atomic positions (Bohr), masses (amu), cell, potential energy and virial over to PLUMED after every MD step, and PLUMED adds the biasing forces in place.

Note: the interface currently supports a single MPI rank only, because PLUMED must see the whole system while ABACUS distributes the atoms of an MD run over the ranks. OpenMP threads inside that single rank are fine, e.g. OMP_NUM_THREADS=32 abacus > log.

example01 runs a well-tempered metadynamics of the ethane (C2H6) H-C-C-H torsion at 600 K (LCAO). Running ABACUS in that directory produces

• COLVAR: time, the torsion angle and the metadynamics bias at every MD step;
• HILLS: the Gaussian hills deposited by metadynamics.

The free-energy surface can be reconstructed with the tools shipped with PLUMED:

plumed sum_hills --hills HILLS --mintozero --outfile fes.dat

fes.dat contains the torsion angle (rad) in the first column and the free energy (kJ/mol) in the second one.

• PLUMED manual: https://www.plumed.org/doc
• ABACUS plumed / plumed_file keywords: https://abacus.deepmodeling.com/en/latest/advanced/input_files/input-main.html