c CCLRC
Section 3.4
3. SHAKE
4. Full step velocity:
v(t)
1
2
v(t -
1
2
t) + v(t +
1
2
t)
(3.37)
5. Thermostat:
(t)
i
v
i
(t) · f
i
(t)
2 E
kin
(t)
.
(3.38)
Several iterations are required to obtain self consistency. In DL POLY 3 the number of iterations
is set to 5 (6 if bond constraints are present).
The conserved quantity by these algorithms is the system kinetic energy.
The VV and LFV flavours of the Gaussian constraints algorithm are implemented in the DL POLY 3
routines nvt e1 vv and nvt e1 lfv respectively.
3.4.3
Berendsen Thermostat
In the Berendsen algorithm the instantaneous temperature is pushed towards the desired temper-
ature T
ext
by scaling the velocities at each step by
(t) = 1 +
t
T
E
kin
(t)
- 1
1/2
,
(3.39)
where
=
f
2
k
B
T
ext
(3.40)
is the target thermostat energy (depending on the external temperature and the system total degrees
of freedom, f - equation (
T
a specified time constant for temperature fluctuations
(normally in the range [0.5, 2] ps).
The VV implementation the algorithm is straight forward. A conventional VV1 and VV2 (thermally
unconstrained) steps are carried out. At the end of VV2 velocities are scaled by a factor of in
the following manner
1. VV1:
v(t +
1
2
t) v(t) +
t
2
f (t)
m
r(t + t) r(t) + t v(t +
1
2
t)
(3.41)
2. RATTLE VV1
3. FF:
f (t + t) f (t)
(3.42)
4. VV2:
v(t + t) v(t +
1
2
t) +
t
2
f (t + t)
m
(3.43)
5. RATTLE VV2
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