---
name: "cceom"
section: "1"
platform: "linux"
package: "psi3"
version: "3.4.0-6+b3"
release: "Debian 12 (Bookworm)"
platforms: ["linux"]
url: "https://new.carta.tech/cceom"
---

# cceom(1)

cceom - equation of motion coupled cluster energy program

**DESCRIPTION**

The program **cceom** calculates EOM-CCSD energies using a Davidson algorithm to solve for the right-hand eigenvectors of the CC Hamiltonian.

**COMMAND LINE ARGUMENTS**

After internal coordinates are detected or generated, optking will enter a run-time mode dependent on command line arguments.

```
--dot_with_Lg    -  check the orthogonality of the excited right-hand eigenvector with
                 -  the ground state left eigenvector, lambda.  cclambda must be run
                 - first for this to work
```

**REFERENCES**

- 1\.

  The equation of motion coupled-cluster method.  A systematic biorthogonal approach to molecular excitation energies, transition probabilities, and excited state properties, J. F. Stanton and R. J. Bartlett, J. Chem. Phys. 98, 7029 (1993).

- **EOM\_REFERENCE =** *integer*

  specifies the algorithm used by cceom.  useful for debugging.  also must currently be set to ROHF, if you want to get triplet excited states from a singlet ground state.  The default is whatever "REFERENCE" is.

- **STATES\_PER\_IRREP =** *array of integers*

  specifies the number of states of each irreducible representation desired.  For example, in C2v symmetry, (0 2 0 0) specifies two excited states of A2 symmetry to be solved, regardless of the symmetry of the ground state.

- **PROP\_SYM =** *integer*

  specifies the symmetry of the state that should be used to compute properties. defaults to the last irrep for which states are requested.

- **PROP\_ROOT =** *integer*

  specifies the number of the root (within its irrep) that should be used to compute properties.  defaults to the highest root requested.

- **EXCITATION\_RANGE =** *excitation\_range*

  used to generate initial guesses.  the larger the value, the more initial guesses are made in the Davidson algorithm.

- **RESIDUAL\_TOL =** *integer*

  Davidson algorithm will converge until the norm of the residual vector is less than 10^(-integer).  default if 4.

- **EVAL\_TOL =** *integer*

  Davidson algorithm will converge until the change in the excitation energy is less than 10^(-integer).  default if 6.