BIRLA INSTITUTE OF TECHNOLOGY AND SCIENCE PILANI
Pilani Campus, Chemistry Department
Chemical Experimentation - II, CHEM F341, Semester II, 2022-23
Laboratory Manual
Experiment:
Computation of Electronic Structure of molecules - II
Duration: 3 h + 3 h
Having done the first experiment on the computation of electronic structure, we get acquainted with
the basics of creation and submission of computational job for electronic structure method. The
theoretical background for this experiment remains the same.
In this experiment, we plan to compute vertical and adiabatic singlet triplet gaps for the given closedshell molecule.
For this, we need to optimize the geometries of the molecule for singlet and triplet states. For
molecules with strong near-degeneracies, methods such as EOM-CC or TD-DFT are required to
compute geometries as well as energies. For non-degenerate or weakly near-degenerate cases, a
simple single reference method such as CCSD, MP2, DFT may also work.
Procedure:
1. Using the Z-matrix generated for the molecule in the previous experiment, create input files
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for geometry optimization+frequency computation for singlet and triplet states of the
molecule.
Submit the opt+frequency jobs and extract the optimized structures (geometries) from the
output files.
Note the zero point vibrational energies and convert it to appropriate units (eV).
Check whether the optimized structure is a local minimum or a transition state.
For each geometry compute the singlet and triplet state energies in two ways – directly using
EOM-SF-CCSD method and indirectly, using the CCSD method.
The Etriplet(singlet geometry)-Esinglet(singlet geometry) is a vertical singlet-triplet gap.
Similarly, Etriplet(triplet geometry)-Esinglet(triplet geometry) is also a vertical singlet-triplet
gap. Physically, what does each of them signify?
For adiabatic gap, the Etriplet(triplet geometry)-Esinglet(singlet geometry) would be required.
Additionally, the zero-point vibrational energy difference must also be added to this to get the
actual energy gap.
For both the structures, note the nuclear repulsion energies.
Represent the singlet triplet gaps graphically. Compile your data and submit the report within
the stipulated period.
DATA TO BE RECORDED: (Append sheets as necessary)
(i)
specifications for geometry optimization jobs: (method, basis set, charge, multiplicity and
starting geometry and the nuclear repulsion energy for the starting structure)
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BIRLA INSTITUTE OF TECHNOLOGY AND SCIENCE PILANI
Pilani Campus, Chemistry Department
Chemical Experimentation - II, CHEM F341, Semester II, 2022-23
Laboratory Manual
(ii)
(iii)
z-matrix for optimized structures (each state), nuclear repulsion energies, zero-point
vibrational energies, number of imaginary frequencies.
Calculations of the energy gaps as instructed in the class.
Students are advised to save the data in a personal storage device for future reference.
Handwritten report on the experiment should be neatly prepared and submitted within the
stipulated period.
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