Geometry input

The geometry is given to ORCA either in the form of Cartesian coordinates (x,y,z) or internal coordinates (bonds, angles, dihedrals). The coordinates can be in the inputfile or in a separate xyz file.

Important: numbering of atoms (and orbitals) in ORCA starts from 0 (instead of 1). This must be remembered when defining constraints for example.


XYZ coordinates in simple format

Cartesian coordinates are by default specified in Ångström units.

!

*xyz 0 1 #Charge and multiplicity (2S+1). Here charge is 0 (neutral),multiplicity=1 (singlet, S=0).
C 0.000000 0.000000 0.000000
O 1.250000 0.000000 0.000000
H -0.587148 0.939049 0.000000
H -0.587148 -0.939049 -0.000000
*

Cartesian coordinates can also be specified in Bohr units instead.

! Bohrs

*xyz 0 1
C 0.000000000 0.000000000 0.000000000
O 2.362157486 0.000000000 0.000000000
H -1.109548835 1.774545300 0.000000000
H -1.109548835 -1.774545300 0.000000000
*

XYZ coordinates via coordinate block

It is also possible to use the %coords block like this:

!

%coords
CTyp xyz # Choose “internal” for Internal coordinates
Charge 0
Mult 1
Unit Angs
coords
C 0.000000 0.000000 0.000000
O 1.250000 0.000000 0.000000
H -0.587148 0.939049 0.000000
H -0.587148 -0.939049 -0.000000
end
end


Internal coordinates (bonds, angles, dihedrals)

Units are in Angstrom and degrees by default.

!
* int 0 1
C 0 0 0 0.0000 0.000 0.00
O 1 0 0 1.2500 0.000 0.00
H 1 2 0 1.1075 122.016 0.00
H 1 2 3 1.1075 122.016 180.00
*


Reading geometry from file

!

* xyzfile 0 1 file.xyz

file.xyz is an xyz file in XMol format with coordinates in Ångström and a 2-line header that contains the number of atoms and a description line:

4
description line
C 0.000000000 0.000000000 0.000000000
O 2.362157486 0.000000000 0.000000000
H -1.109548835 1.774545300 0.000000000
H -1.109548835 -1.774545300 0.000000000


Dummy atoms, ghost atoms, point charges etc.

Sometimes one wants to add additional non-standard parts to the coordinate block i.e dummy atoms.

Dummy atoms: Use "DA" as a symbol and the appropriate coordinates. Dummy atoms are not real atoms, they have no electrons, nucleus or basis functions. They are often used to define constraints.

!

*xyz 0 1
C 0.000000000 0.000000000 0.000000000
O 2.362157486 0.000000000 0.000000000
H -1.109548835 1.774545300 0.000000000
DA 1.000000000 0.000000000 0.000000000 # A dummy atom is defined here. It has no charge or electrons or basis functions.
*

Ghost atoms: Make an atom a ghost atom by adding ":" after its element symbol. A ghost atom has no electrons or nucleus but has basis functions assigned. Ghost atoms are typically used in the counterpoise procedure to correct for basis set superposition error (BSSE). See chapter 5.1.6 in the manual.

!

*xyz 0 1
C 0.000000000 0.000000000 0.000000000
O 2.362157486 0.000000000 0.000000000
H -1.109548835 1.774545300 0.000000000
H: -1.109548835 -1.774545300 0.000000000 # This H atom is defined as a ghost atom.
*

Point charges can also easily be added by both specifying the charge and coordinates and using the symbol "Q". A point charge has no electrons or basis functions but has a fixed charge which may be negative or positive. Point charges in an ORCA calculation are basically handled in the same way as nuclear charges. Note that when adding point charges as "Q" atoms, the self-interaction between point charges is calculated and included in the final energy.

!
#
*xyz 0 1
O 0.000000000 -0.005910000 0.000000000
H 0.765973000 0.587955000 0.000000000
H -0.765973000 0.587955000 0.000000000
Q -0.834 1.374158484 -2.145897424 0.000000000 # Point charge with charge -0.834 is added to xyz position.
Q 0.417 2.328008170 -1.973964147 0.000000000 # Point charge with charge 0.417 is added to xyz position.
Q 0.417 0.980357969 -1.270385486 0.000000000 # Point charge with charge 0.417 is added to xyz position.
*

When many point charges are present, e.g. in QM/MM calculations it is usually more convenient to read in the point charges from a file (see below).

Note that when adding point charges by reading them through a pointcharge file, the self-interaction between point charges is NOT calculated (unlike above).

!
%pointcharges "pointcharges.pc"
*xyz 0 1
O 0.000000000 -0.005910000 0.000000000
H 0.765973000 0.587955000 0.000000000
H -0.765973000 0.587955000 0.000000000
*

where pointcharges.pc contains (first line contains the number of point charges, charge and xyz coordinates (in Angstrom) follow):

3
-0.834 1.374158484 -2.145897424 0.000000000
0.417 2.328008170 -1.973964147 0.000000000
0.417 0.980357969 -1.270385486 0.000000000


Symmetry and orientation

  • For a symmetric (or almost symmetric molecule) one may sometimes want the coordinate system to be oriented in a certain way. This is best done using a visualization program like Chemcraft or Avogadro (or manually) and the oriented coordinates then fed to ORCA. Note that the orientation will not be obeyed during a geometry optimization (as the information is lost during cartesian->internal coordinates conversion) so if a specific orientation is requested, for example for orbital analysis, it makes sense to optimize the coordinates first, then orient the optimized structure outside ORCA and then do a ORCA single-point calculation using the oriented coordinates.

  • ORCA makes limited use of molecular symmetry and is mainly useful for orbital and state analysis in SCF/CASSCF as well as computational advantages in the MRCI code. Using the UseSym keyword it is possible to have ORCA detect the point group, symmetrize the coordinates and change the origin and produce symmetry-adapted orbitals for SCF/CASSCF calculations. Currently, only D2h and lower point groups are supported.

  • In order for the automatic point group detection to work, the molecule needs to have been symmetrized properly before (for example in Chemcraft or Symmetrizer ). It is also possible to change the treshold for pointgroup detection:

! Usesym

%method
SymThresh 5.0e-2
end