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## Main Question or Discussion Point

Dear all,

I am not sure if this is the right place to ask the following. (Moderators, please move this thread to the right sub-forum if necessary.)

The question I am having is related to molecular dynamics. In the book "Molecular Dynamics Simulation: Elementary Methods" by J. M. Haile, a FORTRAN program is provided page 459. A subroutine called SSX2SC is listed and is described as

"the routine to be called only once during a run. At the start of the initialization it is used to scale the initial forces to the program units for acceleration. The scale factor is STPSQH = [tex](\Delta t)^2 / 2[/tex]."

In the FORTRAN program, it goes on to equate:

where X2, Y2 and Z2 are accelerations in x, y and z directions respectively and FX, FY and FZ are forces in x, y and z directions respectively.

The program is treating the problem using reduced units such that mass m = 1, time =[tex]\sigma\sqrt{m/\epsilon}[/tex], energy [tex]\epsilon[/tex] and length [tex]\sigma[/tex].

What I do not understand is:

1. What is the meaning of "to scale the initial forces to the program units for acceleration"?

2. Why multiply force by STPSQH?

Thank you very much and I hope someone can shed some light to this.

Casey

I am not sure if this is the right place to ask the following. (Moderators, please move this thread to the right sub-forum if necessary.)

The question I am having is related to molecular dynamics. In the book "Molecular Dynamics Simulation: Elementary Methods" by J. M. Haile, a FORTRAN program is provided page 459. A subroutine called SSX2SC is listed and is described as

"the routine to be called only once during a run. At the start of the initialization it is used to scale the initial forces to the program units for acceleration. The scale factor is STPSQH = [tex](\Delta t)^2 / 2[/tex]."

In the FORTRAN program, it goes on to equate:

Code:

```
X2(I)=FX(I)*STPSQH
Y2(I)=FY(I)*STPSQH
Z2(I)=FZ(I)*STPSQH
```

The program is treating the problem using reduced units such that mass m = 1, time =[tex]\sigma\sqrt{m/\epsilon}[/tex], energy [tex]\epsilon[/tex] and length [tex]\sigma[/tex].

What I do not understand is:

1. What is the meaning of "to scale the initial forces to the program units for acceleration"?

2. Why multiply force by STPSQH?

Thank you very much and I hope someone can shed some light to this.

Casey

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