How to run the calculations using excited state of DFT level and CCSD(T) in Gaussian? As a example, HONO molecule;
1. How to command the excited state in the rout section in Gaussian?
2. What is the multiplicity of HONO when use as excited state ( 1 or 3,...)? In ground state, it is 1.
3. What is mean of Pre-Reactive-Complex?
It depends on the excited state one need to calculate, for example if the excited state is kind of charge transfer, resulting in multiplicity change, you can change the multiplicity according to the unpaired electrons.
There is another way to calculate excited state using TD-DFT calculation, for example one can calculate triplet excited state using: "td(triplet) b3lyp/6-311g** opt" if you use Gaussian 03 or 09.
It depends on the excited state one need to calculate, for example if the excited state is kind of charge transfer, resulting in multiplicity change, you can change the multiplicity according to the unpaired electrons.
There is another way to calculate excited state using TD-DFT calculation, for example one can calculate triplet excited state using: "td(triplet) b3lyp/6-311g** opt" if you use Gaussian 03 or 09.
In accordance with my experience, you can not calculate excited states at the DFT and CCSD(T) levels in Gaussain. You may use EOMCCSD (equation of motion CCSD) for excited states instead, or CASSCF. Note, that you can calculate with CCSD the lowest electronic state with other multiplicity (the lowest triplet HONO in your case).
Shall I use TD-DFT method for the excited state calculations?
When using TD-DFT, why can not be used singlet (for HONO) such as
# TD B3LYP/6-311G OPT....
1 is multiplicity?
If there is minimum in ground state level, can it convert to transition state in excited state level?
It depends of your problem. My advice for you is that before you start working seriously on your problem, you should read the book "Exploring Chemistry with Electronic Structure Methods". You should get to know with the Configuration Interaction, Complete Active Space and Time-dependent density functional methods. After that everything will be easier for you.
The methods you can use in Gaussian quantum chemical program package is TDDFT, CIS, CIS(D), EOMCCSD, and SAC-CI. As already discussed above, to know which is the best one for your purpose, I recommend you to understand the idea behaind them, i.e. configuration interaction (and coupled-cluster) theory.
The geometry optimized in the ground state is of cause different from those in excited states, so if you'd like to know them you have to perform geometry optimization with the methods given above. Within the methods, TDDFT is the easiest, since TDDFT basically take into account single excitation (even though part of double excitation effects can be treated). But if your interests are in doubly excited state, you have to utilize more sophisticated ones like EOMCCSD or SAC-CI.
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