Re: [AMBER] QM/MM and spin parameter

From: Jason Swails <>
Date: Tue, 3 Jun 2014 11:18:47 -0400

On Tue, Jun 3, 2014 at 10:39 AM, Jinfeng Huang <> wrote:

> Dear Amber users:
> I use Amber11 (AmberTools1.5) to carry out a simulation with odd number
> of electrons in the QM system, but it warned as follows:
> ------------------------Warnings---------------------------------------------
> parameter QMMM: (Spin State) has value 2
> This is outside the legal range
> Lower limit: 1 Upper limit: 1
> The limits may be adjustable; search in the .h files
> ------------------------------------------------------------------------------------
> How can I solve the problem ?

​You can't. The error message states that you can only model singlet

You can interface with an external package (like Gaussian or GAMESS) and
use a method that does model open-shell systems, but you will need to
upgrade to AmberTools 14 for that (and the simulation will be significantly
slower given the increased cost of the QM method you need to use).

Note that open-shell systems are often much more challenging to model
correctly than closed-shell systems, so you may need to do some reading of
the literature to see how others have handled similar problems.


Jason M. Swails
Rutgers University
Postdoctoral Researche
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Received on Tue Jun 03 2014 - 08:30:04 PDT
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