A single reference coupled cluster theory that is capable of including the effect of connected triple excitations has been developed and implemented. This is achieved by regrouping the terms appearing in perturbation theory and parametrizing through two different sets of exponential operators: while one of the exponentials, involving general substitution operators, annihilates the ground state but has a non-vanishing effect when it acts on the excited determinant, the other is the regular single and double excitation operator in the sense of conventional coupled cluster theory, which acts on the Hartree-Fock ground state. The two sets of operators are solved as coupled non-linear equations in an iterative manner without significant increase in computational cost than the conventional coupled cluster theory with singles and doubles excitations. A number of physically motivated and computationally advantageous sufficiency conditions are invoked to arrive at the working equations and have been applied to determine the ground state energies of a number of small prototypical systems having weak multi-reference character. With the knowledge of the correlated ground state, we have reconstructed the triple excitation operator and have performed equation of motion with coupled cluster singles, doubles, and triples to obtain the ionization potential and excitation energies of these molecules as well. Our results suggest that this is quite a reasonable scheme to capture the effect of connected triple excitations as long as the ground state remains weakly multi-reference.

2.
F.
Coester
and
H.
Kummel
,
Nucl. Phys.
17
,
477
(
1960
).
3.
J.
Čížek
,
J. Chem. Phys.
45
,
4256
(
1966
).
4.
J.
Čížek
, “
On the use of the cluster expansion and the technique of diagrams in calculations of correlation effects in atoms and molecules
,” in
Advances in Chemical Physics: Correlation Effects in Atoms and Molecules
, edited by
R.
LeFebvre
and
C.
Moser
(
John Wiley & Sons, Inc., Hoboken, NJ
,
1969
), Vol. 14.
5.
J.
Cizek
and
J.
Paldus
,
Int. J. Quantum Chem.
5
,
359
(
1971
).
6.
J.
Noga
and
R. J.
Bartlett
,
J. Chem. Phys.
86
,
7041
(
1987
).
7.
M.
Urban
,
J.
Noga
,
S. J.
Cole
and
R. J.
Bartlett
,
J. Chem. Phys.
83
,
4041
(
1985
).
8.
J. D.
Watts
,
J.
Gauss
, and
R. J.
Bartlett
,
J. Chem. Phys.
98
,
8718
(
1993
).
9.
K.
Ragavachari
,
G. W.
Trucks
,
J. A.
Pople
, and
M.
Head-Gordon
,
Chem. Phys. Lett.
157
,
479
(
1989
).
10.
R. J.
Bartlett
,
J. D.
Watts
,
S. A.
Kucharski
, and
J.
Noga
,
Chem. Phys. Lett.
165
,
513
(
1990
).
11.
Y. S.
Lee
,
S. A.
Kucharski
, and
R. J.
Bartlett
,
J. Chem. Phys.
81
,
5906
(
1984
).
12.
J.
Noga
,
R. J.
Bartlett
, and
M.
Urban
,
Chem. Phys. Lett.
134
,
126
(
1987
).
13.
R. J.
Bartlett
and
M.
Musial
,
Rev. Mod. Phys.
79
,
291
(
2007
).
14.
D.
Mukherjee
and
S.
Pal
,
Adv. Quantum Chem.
20
,
291
(
1989
).
15.
B.
Jeziorski
and
H. J.
Monkhorst
,
Phys. Rev. A
24
,
1668
(
1981
).
16.
D.
Mukherjee
,
Chem. Phys. Lett.
125
,
207
(
1986
).
17.
U. S.
Mahapatra
,
B.
Datta
, and
D.
Mukherjee
,
Mol. Phys.
94
,
157
(
1998
).
18.
U. S.
Mahapatra
,
B.
Datta
, and
D.
Mukherjee
,
J. Chem. Phys.
110
,
6171
(
1999
).
19.
D. I.
Lyakh
,
M.
Musial
,
V. F.
Lotrich
, and
R. J.
Bartlett
,
Chem. Rev.
112
,
182
(
2012
).
20.
N.
Oliphant
and
L.
Adamowicz
,
J. Chem. Phys.
94
,
1229
(
1991
).
21.
P.
Piecuch
and
L.
Adamowicz
,
J. Chem. Phys.
100
,
5792
(
1994
).
22.
P.
Piecuch
and
L.
Adamowicz
,
J. Chem. Phys.
100
,
5857
(
1994
).
23.
P.
Piecuch
,
S.
Kucharski
,
K.
Kowalski
, and
M.
Musial
,
Comput. Phys. Commun.
149
,
71
(
2002
).
24.
K.
Kowalski
and
P.
Piecuch
,
J. Chem. Phys.
113
,
18
(
2000
).
25.
K.
Kowalski
and
P.
Piecuch
,
J. Chem. Phys.
113
,
5644
(
2000
).
26.
J. J.
Eriksen
,
K.
Kristensen
,
T.
Kjaergaard
,
P.
Jorgensen
, and
J.
Gauss
,
J. Chem. Phys.
140
,
054108
(
2014
).
27.
J. J.
Eriksen
,
D. A.
Matthews
,
P.
Jorgensen
, and
J.
Gauss
,
J. Chem. Phys.
144
,
194102
(
2016
).
28.
S.
Hirata
,
M.
Nooijen
,
I.
Grabowski
, and
R. J.
Bartlett
,
J. Chem. Phys.
114
,
3919
(
2001
).
29.
30.
H.
Nakatsuji
,
Phys. Rev. A
14
,
41
(
1976
).
31.
W.
Kutzelnigg
and
D.
Mukherjee
,
Phys. Rev. A
71
,
022502
(
2005
).
32.
T.
van Voorhis
and
M.
Head-Gordon
,
J. Chem. Phys.
115
,
5033
(
2001
).
33.
M.
Nooijen
and
V.
Lotrich
,
J. Chem. Phys.
113
,
4549
(
2000
).
34.
D. J.
Rowe
,
Rev. Mod. Phys.
40
,
153
(
1968
).
35.
D.
Mukherjee
,
R. K.
Moitra
, and
A.
Mukhopadhyay
,
Mol. Phys.
33
,
955
(
1977
).
36.
A.
Mukhopadhyay
,
R. K.
Moitra
, and
D.
Mukherjee
,
J. Phys. B
12
,
1
(
1979
).
37.
M. A.
Haque
and
D.
Mukherjee
,
J. Chem. Phys.
80
,
5058
(
1984
).
38.
I.
Lindgren
and
D.
Mukherjee
,
Phys. Rep.
151
,
93
(
1987
).
39.
D.
Mukherjee
,
R. K.
Moitra
, and
A.
Mukhopadhyay
,
Mol. Phys.
30
,
1861
(
1975
).
40.
M.
Nooijen
and
R. J.
Bartlett
,
J. Chem. Phys.
106
,
6441
(
1997
).
41.
M.
Nooijen
and
R. J.
Bartlett
,
J. Chem. Phys.
106
,
6449
(
1997
).
42.
M.
Nooijen
and
R. J.
Bartlett
,
J. Chem. Phys.
107
,
6812
(
1997
).
43.
H.
Sekino
and
R. J.
Bartlett
,
Int. J. Quantum Chem. Symp.
26
,
255
(
1984
).
44.
J.
Geertsen
,
M.
Rittby
, and
R. J.
Bartlett
,
Chem. Phys. Lett.
164
,
57
(
1989
).
45.
Y.
Akinaga
and
T.
Nakajima
,
J. Phys. Chem. A
121
,
827
(
2017
).
46.
I.
Lindgren
,
Int. J. Quantum Chem.
14
,
33
(
1978
).
47.
R.
Maitra
,
D.
Sinha
, and
D.
Mukherjee
,
J. Chem. Phys.
137
,
024105
(
2012
).
48.
D.
Sinha
,
R.
Maitra
, and
D.
Mukherjee
,
J. Chem. Phys.
137
,
094104
(
2012
).
49.
J. J.
Phillips
and
D.
Zgid
,
J. Chem. Phys.
140
,
241101
(
2014
).
50.
S. A.
Kucharski
and
R. J.
Bartlett
,
J. Chem. Phys.
108
,
5243
(
1998
).
51.
T.
Nakajima
,
M.
Katouda
,
M.
Kamiya
, and
Y.
Nakatsuka
,
Int. J. Quantum Chem.
115
,
349
(
2015
).
52.
M. W.
Schmidt
,
K. K.
Baldridge
,
J. A.
Boatz
,
S. T.
Elbert
,
M. S.
Gordon
,
J. J.
Jensen
,
S.
Koseki
,
N.
Matsunaga
,
K. A.
Nguyen
,
S.
Su
,
T. L.
Windus
,
M.
Dupuis
, and
J. A.
Montgomery
,
J. Comput. Chem.
14
,
1347
(
1993
).
53.
P.
Piecuch
,
S. A.
Kucharski
, and
V.
Spirko
,
J. Chem. Phys.
111
,
6679
(
1999
).
54.
T. H.
Dunning
, Jr.
,
J. Chem. Phys.
90
,
1007
(
1989
).
55.
T. H.
Dunning
, Jr.
,
J. Chem. Phys.
53
,
2823
(
1970
).
56.
T. H.
Dunning
, Jr.
and
P. J.
Hay
, in
Methods of Electronic Structure Theory
, edited by
H. F.
Schaefer
 III
(
Springer
,
1977
), Vol. 2.
57.
M.
Urban
,
J.
Noga
,
S. J.
Cole
, and
R. J.
Bartlett
,
J. Chem. Phys.
83
,
4041
(
1985
).
58.
K. A.
Peterson
,
J. Chem. Phys.
102
,
262
(
1994
).
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