Missing OPLS-AA parameters for poly styrene-butadiene (PSB) rubber

EMC
1

Hello,

I am building cis-polybutadiene and was missing a torsion parameter in the opls-aa.prm for cm-ct-ct-cm (=CH-CH2-CH2-CH=)
Following the paper of Fröhlich 2013 (https://onlinelibrary.wiley.com/action/downloadSupplement?doi=10.1002%2Fmats.201300103&file=mats201300103-sm-0001-SuppTables.pdf), I suggest adding this line to file field/opls/2012/opls-aa.prm:
cm ct ct cm 1.3 1 0 -0.05 2 0 0.2 3 0

Also, using the OPLS-UA is not preserving the stereochemistry of cis-butadiene even though I put SMILES code “*C/C=C\C*”, only OPLS-AA is preserving. Is there any torsion parameters missing in OPLS-UA too ?

Best,

Eddy BARRAUD

2

I am also modeling styrene butadiene rubber, and OPLS-aa 2012 is missing the definition of styrene cycles. If I try the OPLS-aa 2024 then it is missing torsion parameters:
Warning: no torsion coefficients found for [c4:c4:c3=:h1=].
Warning: no torsion coefficients found for [c3a:c4:c4:c3=].
Warning: no torsion coefficients found for [c3=:c4:c4:c3=].

3

Dear user,

I would suggest using the 2024 instead of the 2012 version of OPLS. The 2012 version uses direct charge assignment, while the 2024 version incorporates bond increments, which makes it a more general force field. I checked out Fröhlich and Sewell, Macromol. Theor. Sim. 2013, 22, 344–353 and noticed, that their suggested dihedral contribution is based on the standard CT-CT-CT-CT or – in 2024 vernacular – c4-c4-c4-c4 (see $EMC_ROOT/field/opls/2024/src/opls-aa.xref for naming convention). I added a generalized rule to the 2024 version (c3*-c4-c4-c*) using the parameters for c4-c4-c4-c4. The resulting 2024 version is given by:

opls-aa.prm (66.7 KB)
opls-aa.top (20.1 KB)

To answer your observation with respect to stereochemistry: EMC forces initial placement of atoms in the correct stereochemistry during the growth process. However, based on the force field, this initial placement could be destroyed during the relaxation step when the accompanying intramolecular structure of the immediate surrounding displays force constants which are unable of maintaining the desired stereochemistry. It seems that this is the case for OPLS-UA.

On a scientific note: styrene-butadiene rubbers seem to consist of a random mix of monomers on the backbone (see Styrene-butadiene - Wikipedia). To simulate this with EMC, one could do the following:

#!/usr/bin/env emc.pl
#
#  script:	setup.esh
#  author:	Pieter J. in 't Veld
#  date:	July 16, 2026
#  purpose:	Example for polymers using EMC setup chemistry mode
#

# Options section

ITEM	OPTIONS

replace		true
field		2024/opls-aa
number		true
ntotal		1000
density		0.1
focus		true
pdb_licorice	true
emc_execute	true

ITEM	END	# OPTIONS

# Groups section

ITEM	GROUPS

# monomers

bd12		*C(C=C)C*, &
		1,bd12:2, 2,bd12:1, &
		1,bd14:2, 2,bd14:1 &
		1,styrene:2, 2,styrene:1, &
		1,term:1, 2,term:1

bd14		*C/C=C\C*, &
		1,bd12:2, 2,bd12:1, &
		1,bd14:2, 2,bd14:1 &
		1,styrene:2, 2,styrene:1, &
		1,term:1, 2,term:1

styrene		*C(c1ccccc1)C*, &
		1,bd12:2, 2,bd12:1, &
		1,bd14:2, 2,bd14:1 &
		1,styrene:2, 2,styrene:1, &
		1,term:1, 2,term:1

term		*C

# polymer

poly		random

ITEM	END	# GROUPS

# Clusters section

ITEM	CLUSTERS

polymer		poly,10

ITEM	END	# CLUSTERS

# Polymers section

ITEM	POLYMERS

poly
log:avg=20,pdi=1.01	bd12:bd14:styrene=1:1:2,$@, term,2

ITEM	END	# POLYMERS

I have added a log-normal distribution to EMC Setup. I also included the above mentioned force field additions. The result I uploaded to https://montecarlo.sourceforge.net/ for the latest version of EMC.