Hello,

it is rather unusual that egrad just stops without further notice during optimization.

The most common problems that occur in excited state optimizations are instabilities of the ground state:

**singlet instabilities,****triplet instabilities,****or non-real instabilities.**

The first two indicate that another electronic state of lower energy exists at the given geometry, a singlet or a triplet state,

respectively. Non-real instabilities (see also the "singlet excited state geometry optimization" post) usually result from conical intersection with the ground state. In all cases it means that the ground state description,e.g., restricted Kohn-Sham, is no longer adequate. In general, this plainly means that TDDFT cannot be applied for the given system at the given geometry.

If an instability occurs in an excited state optimization, you should check several things before giving up:

**check the geometry:** Triplet or non-real instabilities often occur if some bonds become too long, double bonds are

twisted 90 degrees, etc. I could imagine this is what happens with formaldehyde, especially if you start the excited state optimization from the ground state structure.

**look at the orbital occupations:** The system will normally be not stable if there is a "hole" in the orbital occupation, i.e.,

if one or more unoccupied orbitals are lower in energy that some occupied orbitals. Try converging the ground state to an occupation without "holes".**try a ground state calculation:** If the molecule has a triplet instability, you could try a unrelstricted ground state calculation for the triplet state (see "water , triplet excited state geometry optimization" post).**try another functional or Tamm-Dancoff approximation**: Pure density functionals (not including Hartree-Fock

exchange) are much less prone to non-real instabilities that say B3LYP and will work if there are no "holes" in the orbital occupation. The Tamm-Dancoff approximation (specified by $scfinstab ciss/cist/ucis in the control file or in the "ex" menu of define) is said to be more robust with respect to triplet instabilities. However, this does not mean that they are necessarily better for these cases.

There are certainly more possibilities but these might help already.

Greetings,

Dmitrij