RM1 calculations can be very easily carried out in earlier versions of MOPAC by using the keyword AM1 and passing the parameters in file RM1.rm1 via the keyword EXTERNAL, as follows:
- Download, unzip, and copy the file RM1.rm1 to the same folder where your MOPAC input .dat file is;
- In the first line of the input .dat file, use both keywords AM1 and EXTERNAL=RM1.rm1
Use keyword AM1 because RM1 has exactly the same number of parameters as AM1 (see the .dat tab).
That is it! Your calculations will now be MOPAC RM1 calculations.
Although in the output you will find the expression "AM1 CALCULATION RESULTS", the calculation will actually be a MOPAC RM1 calculation. This will happen whenever you use both keywords AM1 and EXTERNAL=RM1.rm1 together.
Warning: whenever you use both keywords AM1 and EXTERNAL=RM1.rm1 together, please make sure that your molecular system contains only atoms of H, C, N, O, P, S, F, CL, Br and I. Otherwise, there will be a mix of RM1 with AM1 parameters which will render the calculation meaningless.