# [gmx-developers] Re: TIP3P and TIP4P parameters

Michael Shirts mrshirts at stanford.edu
Thu Jun 12 06:03:17 CEST 2003

```OK, let me write this all out. The problem comes from the fact that the
original defintions of TIP3P and TIP4P in the 1983 paper in the previous
e-mail come from the equation:

U = A/r^12 - C/r^6

For TIP3P, the constants are A = 5.820 x 10^5, C = 5.950 x 10^2
For TIP4P, the constants are A = 6.000 x 10^5, C = 6.100 x 10^2

Whereas later papers use the form:

U = 4*eps*[(sigma/r)^12 + (sigma/r)^6]

The original paper's values for A and C yield:

TIP3P:
eps = 3.1505651 A     sig = 0.1520725 kcal/mol
eps = 0.31505651 nm   sig = 0.6362713 kj/mol

TIP4P:
eps = 3.1535779 A     sig = 0.1550416 kcal/mol
eps = 0.31535779 nm   sig = 0.6489640 kj/mol

In at least one later paper (Mahoney and Jorgensen, jcp, vol 122 no. 20, 22
May 2000, p. 8910), these parameters are listed as:

TIP3P:
sig = 3.15061 A       eps = 0.1521 kcal/mol

TIP4P:
sig = 3.15365 A       eps = 0.1550 kcal/mol

Clearly there has been rounding, in both the sigma and the epsilon.  Though
the sigma results are a litle strange even then . . .  I've checked these
several times, but if somebody else would once more just to be sure, it would
be nice.

I'll do some test to try to figure out how much difference these changes could
make. Perhaps I'll send an e-mail to Jorgensen and/or Madura about it once I
run those tests . . .

Cheers,
Michael

```