Electronic circular dichroism (ECD) and electronic absorption calculations

 

The calculation of Electronic Circular Dichroism (ECD)   is invoked by the keyword .ECD   in the ABACUS  input module. However, it is also necessary to specify the number of electronic excitations  in each symmetry. As ECD is only observed for chiral molecules, such calculations will in general not employ any symmetry, and a complete input for a molecule without symmetry will thus look like:

**DALTON INPUT
.RUN PROPERTIES
**WAVE FUNCTIONS
.HF
**PROPERTIES
.ECD
*EXCITA
.NEXCITA
    3
*END OF INPUT

In this run we will calculate the rotatory strength  corresponding to the three lowest electronic excitations  (the .NEXCITA   keyword) using London atomic orbitals . If rotatory strengths obtained without London atomic orbitals is also wanted, this is easily accomplished by adding the keyword .ROTVEL   in the *EXCITA   input module.

There has so far only been presented one study of Electronic Circular Dichroism using London atomic orbitals [49], and the results of this investigations indicate that the aug-cc-pVDZ basis set, which is supplied with the DALTON basis set library, is adequate for such calculations.

Another property that may often be of interest is the oscillatory strength . This property can be calculated by an input similar to the one for ECD calculation, and for a molecule with C symmetry an input would look like:

**DALTON INPUT
.RUN PROPERTIES
**WAVE FUNCTIONS
.HF
**PROPERTIES
.EXCITA
*EXCITA
.DIPSTR
.NEXCITA
    3    2    1    0
*END OF DALTON

This input will calculate the dipole strength (.DIPSTR  ) of the 6 lowest electronic excitations distributed in a total of 4 irreducible representations (as in C ). The dipole strength will be calculated both in length and velocity forms. It is expected that the same requirements for basis set quality applies to this property as for ECD.

The two properties may of course be combined a single run, with an input that would then look like (where we also request the rotatory strength to be calculated without the use of London orbitals):

**DALTON INPUT
.RUN PROPERITES
**WAVE FUNCTIONS
.HF
**PROPERTIES
.ECD
.EXCITA
*EXCITA
.DIPSTR
.ROTVEL
.NEXCITA
    3
*END OF INPUT

We also note that excitation energies also can be obtained using the RESPONSE  program (see Chapter ). For a more detailed control of the individual parts of the calculation of properties related to electronic excitation energies, we refer to the input modules affecting the different parts of such calculations:

*EXCITA  

Controls the calculation of electronic excitation energies and the evaluation of all terms contributing to for instance dipole strength or electronic circular dichroism.

*GETSGY  

Controls the setup of the necessary right-hand sides.