AMS biased MD / PLUMED¶

#!/usr/bin/env amspython
# coding: utf-8

# ## Initial imports

import matplotlib.pyplot as plt
import numpy as np
from scm.plams import *

init()


# ## Initial system
#
# Define a Molecule from xyz coordinates and show the molecule.
#
# * O(3) is the right-most O atom
# * H(6) is the left-most H atom


def get_molecule():
    job = AMSJob.from_input(
        """
    system
      Atoms
                  O      -0.1009275285       1.5113007791      -0.4061554537 
                  C       0.0189044656       0.3835929386       0.1570043855
                  O       1.2796450751      -0.2325516597       0.3936038789
                  O      -1.0798994361      -0.4640886294       0.4005134306
                  H       1.7530114719      -0.6822230417      -0.3461237499
                  H      -1.8707340481      -0.5160303870      -0.1988424913
      End
    End
    """
    )
    return job.molecule[""]


mol = get_molecule()

try:
    plot_molecule(mol)  # plot Molecule in Jupyter Notebook in AMS2023+
except NameError:
    pass  # ignore errors in AMS2022-


# ## Calculation settings

current_O3H6 = mol[3].distance_to(mol[6])
target_O3H6 = 0.95

print(f"Scanning bond O3-H6 from {current_O3H6:.3f} to {target_O3H6:.3f} angstrom (this will form a water molecule)")

current_O1C2 = mol[1].distance_to(mol[2])

print(f"Restraining bond O1-C2 at {current_O1C2:.3f} angstrom")


nsteps = 10000  # number of MD steps
kappa = 500000.0  # strength of Plumed MovingRestraint

s = Settings()
# run in serial
s.runscript.nproc = 1
s.runscript.preamble_lines = ["export OMP_NUM_THREADS=1"]

# engine settings
s.input.ReaxFF.ForceField = "CHO.ff"  # If you have ReaxFF license
# s.input.MLPotential.Model = 'M3GNet-UP-2022'   # if you have ML potential license and M3Gnet installed
# s.input.dftb  # if you have a DFTB license

# MD settings
s.input.ams.Task = "MolecularDynamics"
s.input.ams.MolecularDynamics.NSteps = nsteps
s.input.ams.MolecularDynamics.Trajectory.SamplingFreq = 100
s.input.ams.MolecularDynamics.InitialVelocities.Temperature = 200
s.input.ams.MolecularDynamics.Thermostat.Temperature = 500
s.input.ams.MolecularDynamics.Thermostat.Tau = 100
s.input.ams.MolecularDynamics.Thermostat.Type = "Berendsen"

# use an AMS restraint for one of the C-O bond lengths
s.input.ams.Restraints.Distance = []
s.input.ams.Restraints.Distance.append(f"1 2 {current_O1C2} 1.0")

# use an AMS EngineAddon WallPotential to keep the molecules within a sphere of radius 4 angstrom
s.input.ams.EngineAddons.WallPotential.Enabled = "Yes"
s.input.ams.EngineAddons.WallPotential.Radius = 4.0

# Plumed input, note that distances are given in nanometer so multiply by 0.1
s.input.ams.MolecularDynamics.Plumed.Input = f"""
    DISTANCE ATOMS=3,6 LABEL=d36
    MOVINGRESTRAINT ARG=d36 STEP0=1 AT0={current_O3H6*0.1} KAPPA0={kappa} STEP1={nsteps} AT1={target_O3H6*0.1}
    PRINT ARG=d36 FILE=colvar-d36.dat STRIDE=20
    End"""

job = AMSJob(settings=s, molecule=mol, name="dissociating-carbonic-acid")
print(job.get_input())


# ## Run the job

job.run()


# ## Analyze the trajectory
#
# Extract the O3H6 distances at each stored frame, and plot some of the molecules

trajectory = Trajectory(job.results.rkfpath())

every = 20  # picture every 20 frames in the trajectory
N_images = np.int_(np.ceil(len(trajectory) / every))
fig, axes = plt.subplots(1, N_images, figsize=(10, 3))

O3H6_distances = []
i_ax = 0

for i, mol in enumerate(trajectory, 1):
    O3H6_distances.append(mol[3].distance_to(mol[6]))
    if i % every == 1:
        try:
            plot_molecule(mol, ax=axes[i_ax])  # mol is a PLAMS Molecule
            axes[i_ax].set_title(f"frame {i}")
            i_ax += 1
        except NameError:
            pass


# The above pictures show how the H(6) approaches the O(3). At the end, the carbonic acid molecule has dissociated into CO2 and H2O.

plt.plot(O3H6_distances)
plt.ylabel("Distance (angstrom)")
plt.xlabel("Frame")
plt.title("O3-H6 distance")
plt.show()


energies = job.results.get_history_property("Energy")
plt.plot(energies)
plt.ylabel("Energy (hartree)")
plt.show()


# ## A transition state search
#
# PLAMS makes it easy to extract any frame from an MD trajectory. As an example, let's use highest-energy frame as an initial structure for a transition state search with the ADF DFT engine.

index = np.argmax(energies) + 1
approximate_ts_molecule = job.results.get_history_molecule(index)

try:
    plot_molecule(approximate_ts_molecule)
    plt.title(f"Using frame {index} as initial approximate transition state")
except NameError:
    pass


ts_s = Settings()
ts_s.input.ams.task = "TransitionStateSearch"
ts_s.input.ams.GeometryOptimization.InitialHessian.Type = "Calculate"
ts_s.input.ams.Properties.NormalModes = "Yes"
ts_s.input.adf.xc.gga = "PBE"
ts_job = AMSJob(settings=ts_s, molecule=approximate_ts_molecule, name="ts-search")
ts_job.run()


try:
    plot_molecule(ts_job.results.get_main_molecule())
    plt.title("Optimized transition state")
except NameError:
    pass


print("Frequencies (at a TS there should be 1 imaginary [given as negative])")

for f in ts_job.results.get_frequencies():
    print(f"{f:.3f} cm^-1")