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TRIPLET
QUARTET
QUINTET
SEXTET
Even-Electron Systems
If a molecule has an even number of electrons, the
ground state and excited state configurations can be
Singlet, Triplet, or Quintet (not likely). Normally
the ground state is Singlet, but for some molecules,
symmetry considerations indicate a Triplet is the
most stable ground state.
Ground State, RHF
The Ground State, RHF configuration is as follows:
• Singlet ground state—the most common
configuration for a neutral, even electron stable
organic compound. No additional keywords
are necessary.
• Triplet ground state—Use the following
keyword combination: TRIPLET OPEN(2,2)
• Quintet ground state—Use the following
keyword combination: QUINTET OPEN(4,4)
NOTE: The OPEN keyword is normally necessary only
when the molecule has a high degree of symmetry, such as
molecular oxygen. The OPEN keyword increases the active
space available to the SCF calculation by including virtual
orbitals. This is necessary for attaining the higher multiplicity
configurations for even shell system. The OPEN keyword
also invokes the RHF computation using the 1/2 electron
approximation method and a C.I. calculation to correct the
final RHF energies. To see the states used in a C.I.
calculation, type MECI as an additional keyword. The
information is printed at the bottom of the *.out file.
Ground State, UHF
For UHF computations, all unpaired electrons are
forced to be spin up (alpha).
• Singlet ground state—the most common
configuration for a neutral, even electron,
stable organic compound. No additional
keywords are necessary.
• UHF will likely converge to the RHF solution
for Singlet ground states.
• Triplet or Quintet ground state: Use the
keyword TRIPLET or QUINTET.
NOTE: When a higher multiplicity is used, the UHF
solution yields different energies due to separate treatment of
alpha electrons.
Excited State, RHF
First Excited State: The first excited state is actually
the second lowest state (the root=2) for a given
spin system (Singlet, Triplet, Quintet).
To request the first excited state, use the following
sets of keywords:
First excited Singlet: ROOT=2 OPEN(2,2) SINGLET
(or specify the single keyword EXCITED)
First excited triplet: ROOT=2 OPEN (2,2)
TRIPLET C.I.=n, where n=3 is the simplest case.
First excited quintet: ROOT=2 OPEN (4,4)
QUINTET C.I.=n, where n=5 is the simplest case.
Second Excited State: The second excited state is
actually the third lowest state (the root=3) for a
given system (Singlet, Triplet, Quintet). To request
the second excited state use the following set of
keywords:
Second excited Singlet: OPEN(2,2) ROOT=3
SINGLET
Second excited triplet: OPEN(2,2) ROOT=3
TRIPLET C.I.=n, where n=3 is the simplest case.
Second excited quintet: OPEN(4,4) ROOT=3
QUINTET C.I.=n, where n=5 is the simplest case.
170•MOPAC Computations
CambridgeSoft
Specifying the Electronic Configuration