def ra_diff(a, b):
"""Return molecular acceleration difference.
"""
return VecR(a.ra.x - b.ra.x, a.ra.y - b.ra.y, a.ra.z - b.ra.z)
def ComputeForces():
"""Compute the MD forces by evaluating the LJ potential
using cells subdivisions. The simulation area, i.e., region,
is divided into a lattice of small cells and the cell edges all exceed
rcut. For this implementation:
rca = La/Lca, where Lca = FLOOR(La/rcut) (a = x, y, z), and La is
simulation box length in the a direction (region).
+---------------------------------------------+
| | | |
| -1 | 0 Lcx-1|Lcx |
| +---+---+---+---+---+---+---+---+ |
| | | | | |
+------+---+---+---+---+---+---+---+---+------+
"""
cc: VecI = None
m1v: VecI = None
m2v: VecI = None
# Mapping from cell vector to list index (column-major order):
# (c.z * cells.y + c.y) * cells.x + c.x
#
# With cells = VecI(3, 3, 3), list indices are:
vOff = [
VecI(0, 0, 0), # (0 * 3 + 0) * 3 + 0 -> 0
VecI(1, 0, 0), # (0 * 3 + 0) * 3 + 1 -> 1
VecI(1, 1, 0), # (0 * 3 + 1) * 3 + 1 -> 4
VecI(0, 1, 0), # (0 * 3 + 1) * 3 + 0 -> 3
VecI(-1, 1, 0), # (0 * 3 + 1) * 3 - 1 -> 2
VecI(0, 0, 1), # (1 * 3 + 0) * 3 + 0 -> 9
VecI(1, 0, 1), # (1 * 3 + 0) * 3 + 1 -> 10
VecI(1, 1, 1), # (1 * 3 + 1) * 3 + 1 -> 13
VecI(0, 1, 1), # (1 * 3 + 1) * 3 + 0 -> 12
VecI(-1, 1, 1), # (1 * 3 + 1) * 3 - 1 -> 11
VecI(-1, 0, 1), # (1 * 3 + 0) * 3 - 1 -> 8
VecI(-1, -1, 1), # (1 * 3 - 1) * 3 - 1 -> 5
VecI(0, -1, 1), # (1 * 3 - 1) * 3 + 0 -> 6
VecI(1, -1, 1), # (1 * 3 - 1) * 3 + 1 -> 7
]
c: int = 0
j1: int = 0
j2: int = 0
fcVal: float = 0.
uVal: float = 0.
rr: float = 0.
rrCut: float = 0.
rri: float = 0.
rri3: float = 0.
cells = _mdsim_globals['cells']
cellList = _mdsim_globals['cellList']
nCells = cells.vol()
mol = _mdsim_globals['mol']
nMol = _mdsim_globals['nMol']
region = _mdsim_globals['region']
rrCut = _mdsim_globals['rCut'] * _mdsim_globals['rCut']
invWid: VecR = cells / region
for n in range(nMol, nMol + nCells):
cellList[n] = -1
for n in range(nMol):
rs = mol[n].r + (0.5 * region)
cc = VecI(rs.x * invWid.x, rs.y * invWid.y, rs.z * invWid.z)
c = cc.vc_to_list_index(cells) + nMol
cellList[n] = cellList[c]
cellList[c] = n
for m in mol:
m.ra_zero()
_mdsim_globals['uSum'] = 0.
_mdsim_globals['virSum'] = 0.
# Compute cells interactions
shift = VecR()
for m1z in range(cells.z):
for m1y in range(cells.y):
for m1x in range(cells.x):
"""Vector cell index to which this molecule belongs."""
m1v = VecI(m1x, m1y, m1z)
"""Translate vector cell index, m1v, to cell list index."""
m1 = m1v.vc_to_list_index(cells) + nMol
for offset in range(N_OFFSET):
"""Vector cell relative to m1v."""
m2v = m1v + vOff[offset]
""" Periodic boundary-conditions/shift coordinates."""
shift.zero()
_VCell_wrap_all(m2v, cells, shift, region)
"""Translate vector cell index, m2v, to cell list index."""
m2 = m2v.vc_to_list_index(cells) + nMol
j1 = cellList[m1]
while j1 >= 0:
j2 = cellList[m2]
while j2 >= 0:
if m1 != m2 or j2 < j1:
a = mol[j1]
b = mol[j2]
dr = a.r_diff(b)
dr -= shift
rr = vecr_dot(dr, dr)
if rr < rrCut:
rri = 1. / rr
rri3 = rri**3
fcVal = 48.0 * rri3 * (rri3 - 0.5) * rri
uVal = 4. * rri3 * (rri3 - 1.) + 1.
# Molecule at: j1
a.ra_sadd(fcVal, dr)
# Molecule at: j2
b.ra_sadd(-fcVal, dr)
_mdsim_globals['uSum'] += uVal
_mdsim_globals['virSum'] += fcVal * rr
j2 = cellList[j2]
j1 = cellList[j1]
def rv_diff(a, b):
"""Return molecular velocity difference.
"""
return VecR(a.rv.x - b.rv.x, a.rv.y - b.rv.y, a.rv.z - b.rv.z)
