# calculation of volume of a group

Hi,
How to calculate the volume of a specified group when applying some fix command. Is there any command to calculate it.

Hi,
How to calculate the volume of a specified group when applying some fix
command. Is there any command to calculate it.

no. this is a frequently asked question
and the explanation always is that you
are asking to compute an ill-defined quantity.

what is the size of one atom?
how would you define the enclosing surface?

axel.

Hi,
How to calculate the volume of a specified group when applying some fix
command. Is there any command to calculate it.

no. this is a frequently asked question
and the explanation always is that you
are asking to compute an ill-defined quantity.

what is the size of one atom?
how would you define the enclosing surface?

What about using a convex hull construction. Could Qhull (http://www.qhull.org/) be incorporated?

I accept that there would be an error connected to the boundary of the region (which would diminish as the group grew larger), but it's a start.

Nigel

what is the size of one atom?
how would you define the enclosing surface?

What about using a convex hull construction. Could Qhull
(http://www.qhull.org/) be incorporated?

somebody that cares about this enough would need to do it.

there are many ways how this could be done. for example,
you could just place a 3d-grid around the object/group and
compute an occupancy based on an assumed radius for
each atom (type), and then sum up the number of occupied
grid cells and multiply with the volume of each cell.
the question here is, how does one represent enclosed
volume? and when is an object considered "open" or closed.

I accept that there would be an error connected to the boundary of the
region (which would diminish as the group grew larger), but it's a start.

it is not only the size, but also the shape of the group, and the assumed
size of the atoms which makes this an inconsistent property. sure you
can compute a number, but what is the use, when its accuracy depends
on so many different properties?

the vast majority of cases where people have asked for the volume
of a group of atoms have turned out to be bogus, i.e. the real problem
was not the computation of the volume, but the property for which
the volume was required ("local pressure" is the classical example).

cheers,
axel.