[gmx-users] c preprocessor

[Erik Lindahl]

Valentin Gogonea wrote:

> Dear Gromacs users,
>
> I want to added a new atom type to ffgmxnb.itp and it will help me if 
> I can use variables (with #define) for the vdW parameters instead of 
> their values.
>
> In "ffgmxnb.itp" I have:
>
> #define S3c6    0.99844E-02  ; c6  (vdW)
> #define S3c12   0.13078E-04  ; c12 (vdW)
>
> .
> .
> .
>     S    32.06000       0.000       A   0.99844E-02   0.13078E-04
>   S3    32.06000       0.000       A   S3c6          S3c12
>    FE    55.84700       0.000       A   0.00000E+00   0.00000E+00
> .
> .
> .
>
> But I got and error. It sees that cpp doesn't make the substitution. 
> Can anybody suggest me what I should do? The C preprocessor should 
> parse the  "ffgmxnb.itp" file isn't it?

I think the problem is that your S3 line will be preprocessed into:


S3    32.06000       0.000       A   0.99844E-02  ; c6  (vdW)     
 0.13078E-04  ; c12 (vdW)

And everything after the first ";" is (correctly) ignored by grompp.

Try putting the comments on a separate line if you are using defines, i.e:

; c6  (vdW)
#define S3c6    0.99844E-02
 
Cheers,

Erik