[gmx-users] a query....

[Mark Abraham]

Xavier Periole wrote:
> 
>> This is what I got after typing "1 & 2"
>>
>> Copied index group 1 'group1'
>> Copied index group 2 'group2'
>> Merged two groups with AND: 635 637 -> 0
>> Group is empty
>>
> here says the group is empty !!! Which is why there is zero !!!
> the two groups have just no atoms in common !!! If you want to merge
> you got to use pipe "|"

It's Boolean logic applied to sets. "&" is the intersection operator, 
"|" is the union operator.

Mark