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  • DNA force field including e/z OL1 - chi OL4

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This tleap file will load the most recent force field for proteins/DNA/RNA which is called ff14SB.  
Then, it will set as a path the location of or the modified files where the modifications are located. In this case, it is just the name of the directory where we downloaded the file.  Make sure you set the full path.
The addatomtypes is very important because the modifications requires new atom types so add the section exactlyprecisely.
The loadoff command loads the library files of the nucleotides with the new atom types and then the comand command loadamberparams loads the individual modifications, one for the epsilon/zeta dihedral angle and another for a revised version of the chi torsion. In the above example, a PDB file called dna.pdb is loaded to check that the parameters are loaded and everything works correctly.

After this, it is ready, you can just load a PDB with a double stranded DNA and when you generate the AMBER topology and coordinate file, it will be including the e/z OL1 and chi OL4 modifications.

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