Source code for ase.calculators.tip4p

from __future__ import division
import numpy as np

import ase.units as unit
from ase.calculators.calculator import Calculator, all_changes
from ase.calculators.tip3p import rOH, angleHOH, TIP3P

__all__ = ['rOH', 'angleHOH', 'TIP4P', 'sigma0', 'epsilon0']

# Electrostatic constant and parameters:
k_c = 332.1 * unit.kcal / unit.mol
sigma0 = 3.15365
epsilon0 = 0.6480 * unit.kJ / unit.mol


[docs]class TIP4P(TIP3P): def __init__(self, rc=7.0, width=1.0): """ TIP4P potential for water. http://dx.doi.org/10.1063/1.445869 Requires an atoms object of OHH,OHH, ... sequence Correct TIP4P charges and LJ parameters set automatically. Virtual interaction sites implemented in the following scheme: Original atoms object has no virtual sites. When energy/forces are requested: * virtual sites added to temporary xatoms object * energy / forces calculated * forces redistributed from virtual sites to actual atoms object This means you do not get into trouble when propagating your system with MD while having to skip / account for massless virtual sites. This also means that if using for QM/MM MD with GPAW, the EmbedTIP4P class must be used. """ TIP3P.__init__(self, rc, width) self.energy = None self.forces = None def calculate(self, atoms=None, properties=['energy', 'forces'], system_changes=all_changes): Calculator.calculate(self, atoms, properties, system_changes) assert (atoms.numbers[::3] == 8).all() assert (atoms.numbers[1::3] == 1).all() assert (atoms.numbers[2::3] == 1).all() xpos = self.add_virtual_sites(atoms.positions) xcharges = self.get_virtual_charges(atoms) cell = atoms.cell pbc = atoms.pbc natoms = len(atoms) nmol = natoms // 3 self.energy = 0.0 self.forces = np.zeros((4 * natoms // 3, 3)) C = cell.diagonal() assert (cell == np.diag(C)).all(), 'not orthorhombic' assert ((C >= 2 * self.rc) | ~pbc).all(), 'cutoff too large' # Get dx,dy,dz from first atom of each mol to same atom of all other # and find min. distance. Everything moves according to this analysis. for a in range(nmol - 1): D = xpos[(a + 1) * 4::4] - xpos[a * 4] shift = np.zeros_like(D) for i, periodic in enumerate(pbc): if periodic: shift[:, i] = np.rint(D[:, i] / C[i]) * C[i] q_v = xcharges[(a + 1) * 4:] # Min. img. position list as seen for molecule !a! position_list = np.zeros(((nmol - 1 - a) * 4, 3)) for j in range(4): position_list[j::4] += xpos[(a + 1) * 4 + j::4] - shift # Make the smooth cutoff: pbcRoo = position_list[::4] - xpos[a * 4] pbcDoo = np.sum(np.abs(pbcRoo)**2, axis=-1)**(1 / 2) x1 = pbcDoo > self.rc - self.width x2 = pbcDoo < self.rc x12 = np.logical_and(x1, x2) y = (pbcDoo[x12] - self.rc + self.width) / self.width t = np.zeros(len(pbcDoo)) t[x2] = 1.0 t[x12] -= y**2 * (3.0 - 2.0 * y) dtdd = np.zeros(len(pbcDoo)) dtdd[x12] -= 6.0 / self.width * y * (1.0 - y) self.energy_and_forces(a, xpos, position_list, q_v, nmol, t, dtdd) if self.pcpot: e, f = self.pcpot.calculate(xcharges, xpos) self.energy += e self.forces += f f = self.redistribute_forces(self.forces) self.results['energy'] = self.energy self.results['forces'] = f def energy_and_forces(self, a, xpos, position_list, q_v, nmol, t, dtdd): """ energy and forces on molecule a from all other molecules. cutoff is based on O-O Distance. """ # LJ part - only O-O interactions epsil = np.tile([epsilon0], nmol - 1 - a) sigma = np.tile([sigma0], nmol - 1 - a) DOO = position_list[::4] - xpos[a * 4] d2 = (DOO**2).sum(1) d = np.sqrt(d2) e_lj = 4 * epsil * (sigma**12 / d**12 - sigma**6 / d**6) f_lj = (4 * epsil * (12 * sigma**12 / d**13 - 6 * sigma**6 / d**7) * t - e_lj * dtdd)[:, np.newaxis] * DOO / d[:, np.newaxis] self.forces[a * 4] -= f_lj.sum(0) self.forces[(a + 1) * 4::4] += f_lj # Electrostatics e_elec = 0 all_cut = np.repeat(t, 4) for i in range(4): D = position_list - xpos[a * 4 + i] d2_all = (D**2).sum(axis=1) d_all = np.sqrt(d2_all) e = k_c * q_v[i] * q_v / d_all e_elec += np.dot(all_cut, e).sum() e_f = e.reshape(nmol - a - 1, 4).sum(1) F = (e / d_all * all_cut)[:, np.newaxis] * D / d_all[:, np.newaxis] FOO = -(e_f * dtdd)[:, np.newaxis] * DOO / d[:, np.newaxis] self.forces[(a + 1) * 4 + 0::4] += FOO self.forces[a * 4] -= FOO.sum(0) self.forces[(a + 1) * 4:] += F self.forces[a * 4 + i] -= F.sum(0) self.energy += np.dot(e_lj, t) + e_elec def add_virtual_sites(self, pos): # Order: OHHM,OHHM,... # DOI: 10.1002/(SICI)1096-987X(199906)20:8 b = 0.15 xatomspos = np.zeros((4 * len(pos) // 3, 3)) for w in range(0, len(pos), 3): r_i = pos[w] # O pos r_j = pos[w + 1] # H1 pos r_k = pos[w + 2] # H2 pos n = (r_j + r_k) / 2 - r_i n /= np.linalg.norm(n) r_d = r_i + b * n x = 4 * w // 3 xatomspos[x + 0] = r_i xatomspos[x + 1] = r_j xatomspos[x + 2] = r_k xatomspos[x + 3] = r_d return xatomspos def get_virtual_charges(self, atoms): charges = np.empty(len(atoms) * 4 // 3) charges[0::4] = 0.00 # O charges[1::4] = 0.52 # H1 charges[2::4] = 0.52 # H2 charges[3::4] = -1.04 # X1 return charges def redistribute_forces(self, forces): f = forces b = 0.15 a = 0.5 pos = self.atoms.positions for w in range(0, len(pos), 3): r_i = pos[w] # O pos r_j = pos[w + 1] # H1 pos r_k = pos[w + 2] # H2 pos r_ij = r_j - r_i r_jk = r_k - r_j r_d = r_i + b * (r_ij + a * r_jk) / np.linalg.norm(r_ij + a * r_jk) r_id = r_d - r_i gamma = b / np.linalg.norm(r_ij + a * r_jk) x = w * 4 // 3 Fd = f[x + 3] # force on M F1 = (np.dot(r_id, Fd) / np.dot(r_id, r_id)) * r_id Fi = Fd - gamma * (Fd - F1) # Force from M on O Fj = (1 - a) * gamma * (Fd - F1) # Force from M on H1 Fk = a * gamma * (Fd - F1) # Force from M on H2 f[x] += Fi f[x + 1] += Fj f[x + 2] += Fk # remove virtual sites from force array f = np.delete(f, list(range(3, f.shape[0], 4)), axis=0) return f