Excitation energy for triplet

[Lesheng Li]

Dear Lucas and all Qwalk users,

I have a question of how to calculate the first excitation energy for a molecule, in this case CH4. I have done the calculation in the following way:

1. Changed the following parameters from the .slater file

***

ORBITALS { 

....

NMO 4

...

}

STATES { 

#Spin up orbitals

1 2 3 4 

#Spin down orbitals

1 2 3 4

} 

***

to the following:

*** 

ORBITALS { 

....

NMO 5

...

}

STATES { 

#Spin up orbitals

1 2 3 5 

#Spin down orbitals

1 2 3 4

} 

***

By doing this, I am able to obtain the total energy for the modified state vector. 

2. Then, I made a difference of the energies between the value based on the original state vector and the one from the new state vector, delta_E. I think this delta_E is the first excitation energy for CH4. 

Here, I think this is the excitation energy of the singlet, but how can I calculate the first excitation energy for the triplet? Can I just do it by computing the following state vectors?

*** 

ORBITALS { 

....

NMO 5

...

}

STATES { 

#Spin up orbitals

1 2 3 5 5

#Spin down orbitals

1 2 3

} 

***

Thanks,

Lesheng Li

[Lucas Wagner]

Hi Leshing,

The best strategy for computing the singlet-triplet splitting is to just do separate DFT/HF calculations under restricted open shell, one with multiplicity 1, and the other with multiplicity 3. Then run QWalk as normal.

Cheers,

Lucas

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[Lesheng Li]

Hi Lucas,

Thanks for your reply. 

However, when I calculated it under restricted open shell with multiplicity of 3, the converter gives me an error like "main:  error , ndown must be equal to nup". 

When I switched it back to unrestricted, it converts well even through I still have 5 electrons for alpha spin and 3 for beta. 

I have attached the output file from restricted calculation here, could you give me some advices please? 

Thanks,

Lesheng

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[Lucas Wagner]

Hi Leshing,

Sorry we don't really support Gaussian, since I don't have access to the code. GAMESS is much better supported by QWalk.

Lucas

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[Lesheng Li]

Hi Lucas,

Thanks for your reply, I'll have a look at it with GAMESS. 

But is my understanding of the approach for calculating first excitation energy (for the singlet state) correct in my first post? 

Thanks,

Lesheng

[Lucas Wagner]

Hi Leshing,

Your first excitation energy is a reasonable try. You may not obtain a very good answer for such a small molecule because the orbitals typically relax quite a bit in small systems like that. 

You might get a better answer even doing CI-singles to obtain relaxed orbitals. 

Often the singlet-triplet gap is a bit easier because they are ground states of different spin sectors.

Best,

Lucas

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