JMol "calculate HBONDS": which atom is the donor/acceptor?
I dont know about JMol, but this can be easily done with UCSF Chimera.

I loaded the 1a1x structure.
Then selected: Tools > Structure Analysis > FindHBond

I kept the default values and selected Write information to reply log

Then clicked OK 

On the structure the H Bonds will be identified with a coloured line.

To view which is the donor and acceptor go to: Favorites > Reply log

An output of the reply log is below:

H-bonds (donor, acceptor, hydrogen, D..A dist, D-H..A dist):
ASP 12.A N    GLU 25.A OE2  no hydrogen  3.012  N/A
HIS 13.A N    GLU 25.A OE1  no hydrogen  3.338  N/A
HIS 13.A ND1  ASP 12.A OD2  no hydrogen  2.835  N/A
LEU 14.A N    TRP 70.A O    no hydrogen  2.934  N/A
TRP 15.A N    ARG 23.A O    no hydrogen  2.866  N/A
...
120 hydrogen bonds found


As you can see the donor and acceptor has been identified as defined in the header line.

If you wish, the information can also be written to a file by selecteing Write to text file in the FindHBond window.