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The
improper torsion energy of the torsion angle formed
by the atoms i, j, k and l,
Et-ijkl is:
Et-ijkl
= Kt-ijkl[
Tijkl -
To-ijkl
]2
where
T is the torsion angle formed by the ordered
atoms i, j, k and l,
Kt is the force constant and
T0 is the equilibrium
angle.
The
Improper Torsion record must appear only
after the Atom Inclusion Types have been
defined. This record must also appear after the
Atom Inclusion Group record if Groups
are used in this record. The Improper
Torsion record has the following
format:
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:IMPROPER-TORSION
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|
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:Typei:Typej:Typek:Typel:
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Kt-ijkl
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T0-ijkl
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|
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:Typei:Typej:Typek:Typel:
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Kt-ijkl
|
T0-ijkl
|
|
|
...
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...
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...
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:END
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The
opening and closing keywords, like all keywords,
begin with a colon and start on column one of their
respective lines. The body of the data section may
contain any number of lines. A line may be empty or
may contain a list of four Atom Inclusion
Type or Group names followed by two
numbers. The names are written amongst five colons;
the colons are not part of the names. The first
number is the force constant in units of
kcal/mol/radian2. The second number is
the equilibrium torsion angle in degree. Following
the second number may be text to serve as comment.
The comment must be separated, by white space, from
the second number.
An
improper torsion type defined for Inclusion Atom
Types :A:B:C:D: will serve equally well for an
improper torsion of Types :D:C:B:A:. Only
the first instance of a definition of an improper
torsion type will be saved. Subsequent definitions
for the same improper torsion type will be silently
ignored. Therefore, definitions using Groups
should be placed at the end of the list so that the
generic definitions do not override the specific
definitions.
The
program Yup.fpf
will expand Groups into their component
Types, hash each quartet of Types
into a numerical code, and sort the codes. This
forms the index table. The force constants are
converted to internal units used by
YUP
and with the equilibrium torsion angle these
numbers form the data table. The index and data
tables are written to the unformatted file:
"Improper
Torsion Constants".
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