Handbook of Solvents - George Wypych - ChemTech - Ventech!

4.3 Polar solvation dynamics 139
like algorithm. 17 In this representation the mass, M, and the moment of inertia, I, of the solvent
molecules are independent parameters, which makes it possible to study the relative
importance of translational and rotational motions in the solvation process without affecting
other potentially relevant parameters such as the molecular size. The time evolution is done
using the velocity Verlet algorithm, with the value of λ(t) determined as in the SHAKE algorithm,
and with the Andersen 18 thermalization used to keep the system at constant temperature.
For the Stockmayer solvent, the initial molecular parameters are taken to approximate
the CH3Cl molecule: σD = 4.2 Å, εD = 195K,M=50amu, I = 33.54 amu Å 2 , and μ = 1.87 D.
The parameters taken for the solvated ion are MA = 25 amu, σA = 3.675 Å, and εA = 120K, q
is taken to be one electron charge e. These parameters can be changed so as to examine their
effect on the solvation dynamics. Most of the results described below are from simulations
done at 240K, and using L = 33.2Å for the edge length of the cubic simulation cell was, corresponding
to the density ρ = 1.09×10 -2 Å -3 , which is the density of CH3Cl at this tempera-
* 3
ture. In reduced units we have for this choice of parameters ρ ≡ ρσD
= 081, .
*
3
12 /
μ ≡ μ( εDσD) = 132, . T * ≡kBT/εD =1.23, and I * ≡ I(Mσ 2 ) = 0.038. A simple switching
function
f(R) =
�
1
R < Rs
( Rc −R)/( Rc − Rs) Rs < R < Rc
[4.3.31]
0
R > R
is used to smoothly cut off this electrostatic potential. R c and R s are taken to be R c = L/2 and
R s = 0.95R c. Under these simulation conditions the pressure fluctuates in the range 500±100
At.The dielectric constant is computed from pure solvent simulations, using the
expression 33
where
and
( ε− 1)( 2ε′ + 1)
2ε′
+ ε
N
P = ∑ μ i
i = 1
( )
PR
1
=
N
N
1
=
kTR
N
∑∑
c k
j=
1 k=
1
3
B c
( )
PP R
c
c
[4.3.32]
[4.3.33]
'μ [4.3.34]
where the prime on the inner summation indicates the restriction Rjk