# Candidate structures for global minimum determination

What are the best packages that can generate candidate structures for quantum chemical optimization (DFT or CCSD) to find the global minimum? I am working on the gas-phase small inorganic clusters (number of atoms less than 30).

• Does inorganic cluster mean complexes or metal/semiconductor clusters? – Tristan Maxson Aug 10 '20 at 14:11
• @TristanMaxson yes. They are complexes, such as Ag10(H2O)2, Ag(C4H9OH) etc. – Somnath Bhowmick Aug 11 '20 at 12:11

## Confab OpenBabel

source documentation: https://open-babel.readthedocs.io/en/latest/3DStructureGen/multipleconformers.html

Example: obabel <inputfile> -O <outputfile> --confab [confab options]

Some of the options include ecutoff and rcutoff (which help eliminate duplicates)

OpenBabel can be easily downloaded through Anaconda, the python package manager.

However, the energy cutoff criterion may be unreliable for inorganic molecules (you'll have to see) because the molecular mechanics parameters may be unreliable for those elements.

The best bet is to try to generate as many unique conformers as possible and optimize all of them to find which is the lowest energy.

## CCDC

The Cambridge Crystallographic Data Centre organizes a regular organic crystal structure prediction blind test as detailed in their website. These started in 1999 and the latest (the sixth one) was held in 2015. Participants, which include leading research groups in structure prediction, are asked to predict the structure of a range of molecules, the latest test including compounds like a small rigid molecule, drug candidates with multiple polymorphs, and salt hydrates. Although the title of the blind test specifies "organic" crystals, the methods used should be applicable to what you are trying to do.

Each blind test has an associated paper, and you can find the latest here.

• +1. Just edited it to match the format of the other two answers. With 3 highly upvoted answers, I'm surprised this didn't become an HNQ! More upvotes needed for the question and answers. – Nike Dattani Aug 12 '20 at 17:00
• This is an amazing initiative. I wonder how many other fields can adopt this method. – Hitanshu Sachania Oct 18 '20 at 21:44

# xtb

The Grimme group's freely available xtb tight binding program based on the GFN force field can be used to rank candidate structures in energy with the CREST tool.

• +1. And congrats on passing 6000 points! With 2 answers in 2 days, this might become a Hot Network Question :) – Nike Dattani Aug 12 '20 at 6:35