Dear AMBER Developers,
I recently attempted to run QM/MM calculations using *sander-ORCA* with *ORCA
6.1.0*, but ORCA failed to run properly.
After extensive debugging, I found that the issue originates from outdated
ORCA-related settings in the AMBER source code, specifically in:
AmberTools/src/sander/qm2_extern_orc_module.F90
Below I summarize two issues I identified.
------------------------------
*1. Incorrect array dimension for MM coordinates*
At *line 71*, the following declaration appears to be incorrect:
_REAL_, intent(in) :: clcoords(4,nqmatoms) ! MM coordinates and charges in au
Since clcoords stores MM (classical) coordinates and charges, this array
should use nclatoms rather than nqmatoms:
_REAL_, intent(in) :: clcoords(4,nclatoms) ! MM coordinates and charges in au
------------------------------
*2. Deprecated DFT grid keywords incompatible with ORCA 6.1.0*
At *line 408*, the current implementation generates the following ORCA
input block:
if ( dft ) then
! DFT grid info
if ( (orc_nml%grid > 0) .or. (orc_nml%finalgrid > 0) ) then
write(iurun, '(a)') '%method'
if ( orc_nml%grid > 0 ) then
write(iurun, '(a,i1)') ' grid ', orc_nml%grid
end if
if ( orc_nml%finalgrid > 0 ) then
write(iurun, '(a,i1)') ' finalgrid ', orc_nml%finalgrid
end if
write(iurun, '(a)') 'end'
end if
end if
However, the grid and finalgrid keywords are no longer supported in *ORCA
6.1.0*, which causes ORCA to terminate immediately.
Based on the current ORCA documentation, this block should be updated to
use the newer %method syntax, for example:
if ( dft ) then
! DFT grid info
if ( (orc_nml%grid > 0) .or. (orc_nml%finalgrid > 0) ) then
write(iurun, '(a)') '%method'
if ( orc_nml%grid > 0 ) then
write(iurun, '(a)') ' AngularGrid 4'
end if
if ( orc_nml%finalgrid > 0 ) then
write(iurun, '(a)') ' IntAcc 4.5'
write(iurun, '(a)') ' AngularGridX 1,2,3'
write(iurun, '(a)') ' HGridReduced true'
end if
write(iurun, '(a)') 'end'
end if
end if
Alternatively, these legacy grid-related lines could be commented out
entirely, and users could rely on ORCA’s predefined grid settings (e.g.,
DEFGRID1 or DEFGRID2) specified via the cbasis / jcasis options, which
appear to be fully compatible with ORCA 6.x.
------------------------------
I hope this information is helpful and may assist in improving
compatibility between AMBER and newer versions of ORCA.
Thank you very much for your time and for maintaining AMBER.
Best regards,
Jun-Min Liao
Postdoctoral Researcher
National Cheng Kung University
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Received on Fri Dec 19 2025 - 07:00:03 PST
