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.