NWChem computer exercise

Quantumchemistry, NWI-MOL406, Computer exercises, Assignment 1

In this exerise we study a simple chemical reaction of the S_N2 type:

Cl⁻ + CH₃Br → CH₃Cl + Br⁻

This includes:

Structure of reactants and products
Pre/post-reactive complex
Transition state

Note that this system has a very simple reaction coordinate. You can use the appropriate tool in NWCHEM and constrain your search so that the system has three-fold symmetry, e.g., by using a "hand-written" z-matrix. You could also use xyz-format but take care that C_3v symmetry is present in you initial configuration. Look in the NWChem manual ( https://github.com/nwchemgit/nwchem/wiki or https://nwchemgit.github.io) , e.g., at "saddle", on how to do a transition state (TS) search. You may have to start the NWCHEM TS search with an initial guessed configuration already close to the TS. Also, you may want to do a frequency calculation first, in order to select the mode that is most similar to the reaction coordinate. But with a judicious choice of starting structure, you might be able to avoid that.

For this exercise use B3LYP and investigate the effect of basis set (in)completeness:

3-21++G* for H and C, 3-21g for Br and Cl
6-311++G** for H and C, 6-311G** for Br and Cl

Compare your result with accurate quantum chemical calculations from the litterature.

Please also see the checklist.

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