@article{andersson_1990,
  title = {Second-Order Perturbation Theory with a {{CASSCF}} Reference Function},
  author = {Andersson, Kerstin. and Malmqvist, Per Aake. and Roos, Bjoern O. and Sadlej, Andrzej J. and Wolinski, Krzysztof.},
  year = {1990},
  month = jul,
  volume = {94},
  pages = {5483--5488},
  publisher = {{American Chemical Society}},
  issn = {0022-3654},
  doi = {10.1021/j100377a012},
  file = {/Users/monino/Zotero/storage/5LW6PKJ9/Andersson et al. - 1990 - Second-order perturbation theory with a CASSCF ref.pdf;/Users/monino/Zotero/storage/VXS655QG/j100377a012.html},
  journal = {J. Phys. Chem.},
  number = {14}
}

@article{angeli_2001a,
  title = {Introduction of N-Electron Valence States for Multireference Perturbation Theory},
  author = {Angeli, C. and Cimiraglia, R. and Evangelisti, S. and Leininger, T. and Malrieu, J.-P.},
  year = {2001},
  month = jun,
  volume = {114},
  pages = {10252--10264},
  publisher = {{American Institute of Physics}},
  issn = {0021-9606},
  doi = {10.1063/1.1361246},
  file = {/Users/monino/Zotero/storage/LXLLFJXM/Angeli et al. - 2001 - Introduction of n-electron valence states for mult.pdf},
  journal = {J. Chem. Phys.},
  number = {23}
}

@article{angeli_2001b,
  title = {N-Electron Valence State Perturbation Theory: A Fast Implementation of the Strongly Contracted Variant},
  shorttitle = {N-Electron Valence State Perturbation Theory},
  author = {Angeli, Celestino and Cimiraglia, Renzo and Malrieu, Jean-Paul},
  year = {2001},
  month = dec,
  volume = {350},
  pages = {297--305},
  issn = {0009-2614},
  doi = {10.1016/S0009-2614(01)01303-3},
  abstract = {In this work we reconsider the strongly contracted variant of the n-electron valence state perturbation theory (SC NEV-PT) which uses Dyall's Hamiltonian to define the zero-order energies (SC NEV-PT(D)). We develop a formalism in which the key quantities used for the second-order perturbation correction to the energy are written in terms of the matrix elements of suitable operators evaluated on the zero-order wavefunction, without the explicit knowledge of the perturbation functions. The new formalism strongly improves the computation performances. As test cases we present two preliminary studies: (a) on N2 where the convergence of the spectroscopic properties as a function of the basis set and CAS-CI space is discussed and (b) on Cr2 where it is shown that the SC NEV-PT(D) method is able to provide the correct profile for the potential energy curve.},
  file = {/Users/monino/Zotero/storage/MU8H53BC/Angeli et al. - 2001 - N-electron valence state perturbation theory a fa.pdf;/Users/monino/Zotero/storage/KW4GRB2F/S0009261401013033.html},
  journal = {Chemical Physics Letters},
  language = {en},
  number = {3}
}

@article{angeli_2002,
  title = {N-Electron Valence State Perturbation Theory: {{A}} Spinless Formulation and an Efficient Implementation of the Strongly Contracted and of the Partially Contracted Variants},
  shorttitle = {N-Electron Valence State Perturbation Theory},
  author = {Angeli, Celestino and Cimiraglia, Renzo and Malrieu, Jean-Paul},
  year = {2002},
  month = nov,
  volume = {117},
  pages = {9138--9153},
  publisher = {{American Institute of Physics}},
  issn = {0021-9606},
  doi = {10.1063/1.1515317},
  file = {/Users/monino/Zotero/storage/HHFA46GF/Angeli et al. - 2002 - n-electron valence state perturbation theory A sp.pdf;/Users/monino/Zotero/storage/CPKUV9TE/1.html},
  journal = {J. Chem. Phys.},
  number = {20}
}

@book{AromaticityAntiaromaticityElectronic,
  title = {Aromaticity and {{Antiaromaticity}}: {{Electronic}} and {{Structural Aspects}} | {{Wiley}}},
  shorttitle = {Aromaticity and {{Antiaromaticity}}},
  file = {/Users/monino/Zotero/storage/HGW4QMJY/Aromaticity+and+Antiaromaticity+Electronic+and+Structural+Aspects-p-9780471593829.html}
}

@article{baeyer_1885,
  title = {Ueber {{Polyacetylenverbindungen}}},
  author = {Baeyer, Adolf},
  year = {1885},
  volume = {18},
  pages = {2269--2281},
  issn = {1099-0682},
  doi = {10.1002/cber.18850180296},
  annotation = {\_eprint: https://chemistry-europe.onlinelibrary.wiley.com/doi/pdf/10.1002/cber.18850180296},
  copyright = {Copyright \textcopyright{} 1885 WILEY-VCH Verlag GmbH \& Co. KGaA, Weinheim},
  file = {/Users/monino/Zotero/storage/T9A8FP8V/Baeyer - 1885 - Ueber Polyacetylenverbindungen.pdf;/Users/monino/Zotero/storage/B56CA56Z/cber.html},
  journal = {Berichte Dtsch. Chem. Ges.},
  language = {en},
  number = {2}
}

@article{bally_1980,
  title = {Cyclobutadiene},
  author = {Bally, Thomas and Masamune, Satoru},
  year = {1980},
  month = jan,
  volume = {36},
  pages = {343--370},
  issn = {0040-4020},
  doi = {10.1016/0040-4020(80)87003-7},
  file = {/Users/monino/Zotero/storage/DXWL3L8N/Bally et Masamune - 1980 - Cyclobutadiene.pdf;/Users/monino/Zotero/storage/XQ98S2QN/0040402080870037.html},
  journal = {Tetrahedron},
  language = {en},
  number = {3}
}

@article{casanova_2020,
  title = {Spin-Flip Methods in Quantum Chemistry},
  author = {Casanova, David and Krylov, Anna I.},
  year = {2020},
  month = feb,
  volume = {22},
  pages = {4326--4342},
  publisher = {{The Royal Society of Chemistry}},
  issn = {1463-9084},
  doi = {10.1039/C9CP06507E},
  abstract = {This Perspective discusses salient features of the spin-flip approach to strong correlation and describes different methods that sprung from this idea. The spin-flip treatment exploits the different physics of low-spin and high-spin states and is based on the observation that correlation is small for same-spin electrons. By using a well-behaved high-spin state as a reference, one can access problematic low-spin states by deploying the same formal tools as in the excited-state treatments (i.e., linear response, propagator, or equation-of-motion theories). The Perspective reviews applications of this strategy within wave function and density functional theory frameworks as well as the extensions for molecular properties and spectroscopy. The utility of spin-flip methods is illustrated by examples. Limitations and proposed future directions are also discussed.},
  file = {/Users/monino/Zotero/storage/7E3MQEQM/Casanova et Krylov - 2020 - Spin-flip methods in quantum chemistry.pdf},
  journal = {Phys. Chem. Chem. Phys.},
  language = {en},
  number = {8}
}

@article{christiansen_1995,
  title = {Response Functions in the {{CC3}} Iterative Triple Excitation Model},
  author = {Christiansen, Ove and Koch, Henrik and Jo/rgensen, Poul},
  year = {1995},
  month = nov,
  volume = {103},
  pages = {7429--7441},
  publisher = {{American Institute of Physics}},
  issn = {0021-9606},
  doi = {10.1063/1.470315},
  journal = {J. Chem. Phys.},
  number = {17}
}

@article{eckert-maksic_2006,
  title = {Automerization Reaction of Cyclobutadiene and Its Barrier Height: {{An}} Ab Initio Benchmark Multireference Average-Quadratic Coupled Cluster Study},
  shorttitle = {Automerization Reaction of Cyclobutadiene and Its Barrier Height},
  author = {{Eckert-Maksi{\'c}}, Mirjana and Vazdar, Mario and Barbatti, Mario and Lischka, Hans and Maksi{\'c}, Zvonimir B.},
  year = {2006},
  month = aug,
  volume = {125},
  pages = {064310},
  publisher = {{American Institute of Physics}},
  issn = {0021-9606},
  doi = {10.1063/1.2222366},
  abstract = {The problem of the double bond flipping interconversion of the two equivalent ground state structures of cyclobutadiene (CBD) is addressed at the multireference average-quadratic coupled cluster level of theory, which is capable of optimizing the structural parameters of the ground, transition, and excited states on an equal footing. The barrier height involving both the electronic and zero-point vibrational energy contributions is 6.3kcalmol-16.3kcalmol-1{$<$}math display="inline" overflow="scroll" altimg="eq-00001.gif"{$><$}mrow{$><$}mn{$>$}6.3{$<$}/mn{$><$}mspace width="0.3em"{$><$}/mspace{$><$}mi{$>$}kcal{$<$}/mi{$><$}mspace width="0.2em"{$><$}/mspace{$><$}msup{$><$}mi{$>$}mol{$<$}/mi{$><$}mrow{$><$}mo{$>-<$}/mo{$><$}mn{$>$}1{$<$}/mn{$><$}/mrow{$><$}/msup{$><$}/mrow{$><$}/math{$>$}, which is higher than the best earlier theoretical estimate of 4.0kcalmol-14.0kcalmol-1{$<$}math display="inline" overflow="scroll" altimg="eq-00002.gif"{$><$}mrow{$><$}mn{$>$}4.0{$<$}/mn{$><$}mspace width="0.3em"{$><$}/mspace{$><$}mi{$>$}kcal{$<$}/mi{$><$}mspace width="0.2em"{$><$}/mspace{$><$}msup{$><$}mi{$>$}mol{$<$}/mi{$><$}mrow{$><$}mo{$>-<$}/mo{$><$}mn{$>$}1{$<$}/mn{$><$}/mrow{$><$}/msup{$><$}/mrow{$><$}/math{$>$}. This result is confirmed by including into the reference space the orbitals of the CC {$\sigma\sigma<$}math display="inline" overflow="scroll" altimg="eq-00003.gif"{$><$}mi{$>\sigma<$}/mi{$><$}/math{$>$} bonds beyond the standard {$\pi\pi<$}math display="inline" overflow="scroll" altimg="eq-00004.gif"{$><$}mi{$>\pi<$}/mi{$><$}/math{$>$} orbital space. It places the present value into the middle of the range of the measured data (1.6\textendash 10kcalmol-1)(1.6\textendash 10kcalmol-1){$<$}math display="inline" overflow="scroll" altimg="eq-00005.gif"{$><$}mrow{$><$}mo{$>$}({$<$}/mo{$><$}mn{$>$}1.6{$<$}/mn{$><$}mo{$>$}\textendash{$<$}/mo{$><$}mn{$>$}10{$<$}/mn{$><$}mspace width="0.3em"{$><$}/mspace{$><$}mi{$>$}kcal{$<$}/mi{$><$}mspace width="0.2em"{$><$}/mspace{$><$}msup{$><$}mi{$>$}mol{$<$}/mi{$><$}mrow{$><$}mo{$>-<$}/mo{$><$}mn{$>$}1{$<$}/mn{$><$}/mrow{$><$}/msup{$><$}mo{$>$}){$<$}/mo{$><$}/mrow{$><$}/math{$>$}. An adiabatic singlet-triplet energy gap of 7.4kcalmol-17.4kcalmol-1{$<$}math display="inline" overflow="scroll" altimg="eq-00006.gif"{$><$}mrow{$><$}mn{$>$}7.4{$<$}/mn{$><$}mspace width="0.3em"{$><$}/mspace{$><$}mi{$>$}kcal{$<$}/mi{$><$}mspace width="0.2em"{$><$}/mspace{$><$}msup{$><$}mi{$>$}mol{$<$}/mi{$><$}mrow{$><$}mo{$>-<$}/mo{$><$}mn{$>$}1{$<$}/mn{$><$}/mrow{$><$}/msup{$><$}/mrow{$><$}/math{$>$} between the transition state Btg1Btg1{$<$}math display="inline" overflow="scroll" altimg="eq-00007.gif"{$><$}mmultiscripts{$><$}mi{$>$}B{$<$}/mi{$><$}mrow{$><$}mi{$>$}t{$<$}/mi{$><$}mi{$>$}g{$<$}/mi{$><$}/mrow{$><$}none{$><$}/none{$><$}mprescripts{$><$}/mprescripts{$><$}none{$><$}/none{$><$}mn{$>$}1{$<$}/mn{$><$}/mmultiscripts{$><$}/math{$>$} and the first triplet A2g3A2g3{$<$}math display="inline" overflow="scroll" altimg="eq-00008.gif"{$><$}mmultiscripts{$><$}mi{$>$}A{$<$}/mi{$><$}mrow{$><$}mn{$>$}2{$<$}/mn{$><$}mi{$>$}g{$<$}/mi{$><$}/mrow{$><$}none{$><$}/none{$><$}mprescripts{$><$}/mprescripts{$><$}none{$><$}/none{$><$}mn{$>$}3{$<$}/mn{$><$}/mmultiscripts{$><$}/math{$>$} state is obtained. A low barrier height for the CBD automerization and a small {$\Delta$}E(A2g3,B1g1){$\Delta$}E(A2g3,B1g1){$<$}math display="inline" overflow="scroll" altimg="eq-00009.gif"{$><$}mrow{$><$}mi{$>\Delta<$}/mi{$><$}mi{$>$}E{$<$}/mi{$><$}mrow{$><$}mo{$>$}({$<$}/mo{$><$}mmultiscripts{$><$}mi{$>$}A{$<$}/mi{$><$}mrow{$><$}mn{$>$}2{$<$}/mn{$><$}mi{$>$}g{$<$}/mi{$><$}/mrow{$><$}none{$><$}/none{$><$}mprescripts{$><$}/mprescripts{$><$}none{$><$}/none{$><$}mn{$>$}3{$<$}/mn{$><$}/mmultiscripts{$><$}mo{$>$},{$<$}/mo{$><$}mmultiscripts{$><$}mi{$>$}B{$<$}/mi{$><$}mrow{$><$}mn{$>$}1{$<$}/mn{$><$}mi{$>$}g{$<$}/mi{$><$}/mrow{$><$}none{$><$}/none{$><$}mprescripts{$><$}/mprescripts{$><$}none{$><$}/none{$><$}mn{$>$}1{$<$}/mn{$><$}/mmultiscripts{$><$}mo{$>$}){$<$}/mo{$><$}/mrow{$><$}/mrow{$><$}/math{$>$} gap bear some relevance on the highly pronounced reactivity of CBD, which is briefly discussed.},
  file = {/Users/monino/Zotero/storage/F5Y4YKWD/Eckert-Maksić et al. - 2006 - Automerization reaction of cyclobutadiene and its .pdf;/Users/monino/Zotero/storage/SSRES9DP/1.html},
  journal = {J. Chem. Phys.},
  number = {6}
}

@article{ermer_1983,
  title = {Three {{Arguments Supporting}} a {{Rectangular Structure}} for {{Tetra}}-Tert-Butylcyclobutadiene},
  author = {Ermer, Otto and Heilbronner, Edgar},
  year = {1983},
  volume = {22},
  pages = {402--403},
  issn = {1521-3773},
  doi = {10.1002/anie.198304021},
  annotation = {\_eprint: https://onlinelibrary.wiley.com/doi/pdf/10.1002/anie.198304021},
  copyright = {Copyright \textcopyright{} 1983 by Verlag Chemie, GmbH, Germany},
  file = {/Users/monino/Zotero/storage/T32BDQPQ/Ermer et Heilbronner - 1983 - Three Arguments Supporting a Rectangular Structure.pdf;/Users/monino/Zotero/storage/4BR2A634/anie.html},
  journal = {Angew. Chem. Int. Ed. Engl.},
  language = {en},
  number = {5}
}

@article{hirata_2000,
  title = {High-Order Determinantal Equation-of-Motion Coupled-Cluster Calculations for Electronic Excited States},
  author = {Hirata, So and Nooijen, Marcel and Bartlett, Rodney J.},
  year = {2000},
  month = aug,
  volume = {326},
  pages = {255--262},
  issn = {0009-2614},
  doi = {10.1016/S0009-2614(00)00772-7},
  abstract = {A general-order equation-of-motion coupled-cluster (EOM-CC) method, which is capable of computing the excitation energies of molecules at any given pair of orders (m and n) of the cluster operator and the linear excitation operator, is developed by employing a determinantal algorithm. The EOM-CC(m,n) results of the vertical excitation energies are presented for CH+ with m and n varied independently in the range of 1{$\leqslant$}m,n{$\leqslant$}4 and for CH2 with 1{$\leqslant$}m=n{$\leqslant$}6. EOM-CCSDT [EOM-CC(3,3)] provides the excitation energies that are within 0.1 eV of the full configuration interaction results for dominant double replacement transitions.},
  file = {/Users/monino/Zotero/storage/ZZI4JPPT/Hirata et al. - 2000 - High-order determinantal equation-of-motion couple.pdf},
  journal = {Chemical Physics Letters},
  language = {en},
  number = {3}
}

@article{hirata_2004,
  title = {Higher-Order Equation-of-Motion Coupled-Cluster Methods},
  author = {Hirata, So},
  year = {2004},
  month = jul,
  volume = {121},
  pages = {51--59},
  publisher = {{American Institute of Physics}},
  issn = {0021-9606},
  doi = {10.1063/1.1753556},
  file = {/Users/monino/Zotero/storage/3HCJINRI/Hirata - 2004 - Higher-order equation-of-motion coupled-cluster me.pdf},
  journal = {J. Chem. Phys.},
  number = {1}
}

@article{irngartinger_1983,
  title = {Bonding {{Electron Density Distribution}} in {{Tetra}}-Tert-Butylcyclobutadiene\textemdash{} {{A Molecule}} with an {{Obviously Non}}-{{Square Four}}-{{Membered}} Ring},
  author = {Irngartinger, Hermann and Nixdorf, Matthias},
  year = {1983},
  volume = {22},
  pages = {403--404},
  issn = {1521-3773},
  doi = {10.1002/anie.198304031},
  annotation = {\_eprint: https://onlinelibrary.wiley.com/doi/pdf/10.1002/anie.198304031},
  copyright = {Copyright \textcopyright{} 1983 by Verlag Chemie, GmbH, Germany},
  file = {/Users/monino/Zotero/storage/QZP8JWNP/Irngartinger et Nixdorf - 1983 - Bonding Electron Density Distribution in Tetra-ter.pdf;/Users/monino/Zotero/storage/X5NU6NTT/anie.html},
  journal = {Angew. Chem. Int. Ed. Engl.},
  language = {en},
  number = {5}
}

@article{kallay_2004,
  title = {Calculation of Excited-State Properties Using General Coupled-Cluster and Configuration-Interaction Models},
  author = {K{\'a}llay, Mih{\'a}ly and Gauss, J{\"u}rgen},
  year = {2004},
  month = nov,
  volume = {121},
  pages = {9257--9269},
  publisher = {{American Institute of Physics}},
  issn = {0021-9606},
  doi = {10.1063/1.1805494},
  file = {/Users/monino/Zotero/storage/TEHKUF6P/Kállay et Gauss - 2004 - Calculation of excited-state properties using gene.pdf},
  journal = {J. Chem. Phys.},
  number = {19}
}

@article{koch_1997,
  title = {The {{CC3}} Model: {{An}} Iterative Coupled Cluster Approach Including Connected Triples},
  shorttitle = {The {{CC3}} Model},
  author = {Koch, Henrik and Christiansen, Ove and Jo/rgensen, Poul and {Sanchez de Mer{\'a}s}, Alfredo M. and Helgaker, Trygve},
  year = {1997},
  month = feb,
  volume = {106},
  pages = {1808--1818},
  publisher = {{American Institute of Physics}},
  issn = {0021-9606},
  doi = {10.1063/1.473322},
  file = {/Users/monino/Zotero/storage/BEW2ATM3/Koch et al. - 1997 - The CC3 model An iterative coupled cluster approa.pdf},
  journal = {J. Chem. Phys.},
  number = {5}
}

@article{kreile_1986,
  title = {Uv Photoelectron Spectrum of Cyclobutadiene. Free Cyclobutadiene Stable up to High Temperatures},
  author = {Kreile, J{\"u}rgen and M{\"u}nzel, Norbert and Schweig, Armin and Specht, Harald},
  year = {1986},
  month = feb,
  volume = {124},
  pages = {140--146},
  issn = {0009-2614},
  doi = {10.1016/0009-2614(86)85133-8},
  abstract = {The Hel photoelectron spectrum of cyclobutadiene (CB) has been obtained under conditions which demonstrate that free CB is stable up to temperatures of several hundred \textdegree C. A new experimental argument for the rectangular geometry of CB is presented. Shake-up structures are unimportant for the interpretation of the PE spectrum of CB. LNDO/S PERTCI, MNDO PERTCI and previous experimental vertical ionization energy estimates accord with the experimental data.},
  file = {/Users/monino/Zotero/storage/2EQ8LH4G/Kreile et al. - 1986 - Uv photoelectron spectrum of cyclobutadiene. free .pdf;/Users/monino/Zotero/storage/QHJZT5VV/0009261486851338.html},
  journal = {Chemical Physics Letters},
  language = {en},
  number = {2}
}

@article{kucharski_1991,
  title = {Recursive Intermediate Factorization and Complete Computational Linearization of the Coupled-Cluster Single, Double, Triple, and Quadruple Excitation Equations},
  author = {Kucharski, Stanislaw A. and Bartlett, Rodney J.},
  year = {1991},
  month = jul,
  volume = {80},
  pages = {387--405},
  issn = {1432-2234},
  doi = {10.1007/BF01117419},
  abstract = {The nonlinear CCSDTQ equations are written in a fully linearized form, via the introduction of computationally convenient intermediates. An efficient formulation of the coupled cluster method is proposed. Due to a recursive method for the calculation of intermediates, all computational steps involve the multiplication of an intermediate with aT vertex. This property makes it possible to express the CC equations exclusively in terms of matrix products which can be directly transformed into a highly vectorized program.},
  file = {/Users/monino/Zotero/storage/L3VLAU8A/Kucharski et Bartlett - 1991 - Recursive intermediate factorization and complete .pdf},
  journal = {Theoret. Chim. Acta},
  language = {en},
  number = {4}
}

@article{reeves_1969,
  title = {Further Experiments Pertaining to the Ground State of Cyclobutadiene},
  author = {Reeves, P. C. and Henery, J. and Pettit, R.},
  year = {1969},
  month = oct,
  volume = {91},
  pages = {5888--5890},
  publisher = {{American Chemical Society}},
  issn = {0002-7863},
  doi = {10.1021/ja01049a042},
  file = {/Users/monino/Zotero/storage/T44XQHXX/Reeves et al. - 1969 - Further experiments pertaining to the ground state.pdf;/Users/monino/Zotero/storage/YFJV7DYC/ja01049a042.html},
  journal = {J. Am. Chem. Soc.},
  number = {21}
}

@book{roos_1996,
  title = {Multiconfigurational {{Perturbation Theory}}: {{Applications}} in {{Electronic Spectroscopy}}},
  shorttitle = {Multiconfigurational {{Perturbation Theory}}},
  booktitle = {Advances in {{Chemical Physics}}},
  author = {Roos, Bj{\"o}rn O. and Andersson, Kerstin and F{\"u}lscher, Markus P. and Malmqvist, Per-{\^a}ke and {Serrano-Andr{\'e}s}, Luis and Pierloot, Kristin and Merch{\'a}n, Manuela},
  year = {1996},
  pages = {219--331},
  publisher = {{John Wiley \& Sons, Ltd}},
  doi = {10.1002/9780470141526.ch5},
  abstract = {This chapter contains sections titled: Introduction Multiconfigurational Perturbation Theory Applications in Spectroscopy Summary},
  annotation = {\_eprint: https://onlinelibrary.wiley.com/doi/pdf/10.1002/9780470141526.ch5},
  copyright = {Copyright \textcopyright{} 1996 by John Wiley \& Sons, Inc.},
  file = {/Users/monino/Zotero/storage/KWDFZUBF/9780470141526.html},
  isbn = {978-0-470-14152-6}
}

@article{whitman_1982,
  title = {Limits on the Activation Parameters for Automerization of Cyclobutadiene-1,2-D2},
  author = {Whitman, David W. and Carpenter, Barry K.},
  year = {1982},
  month = nov,
  volume = {104},
  pages = {6473--6474},
  publisher = {{American Chemical Society}},
  issn = {0002-7863},
  doi = {10.1021/ja00387a065},
  file = {/Users/monino/Zotero/storage/9AK8SNDG/Whitman et Carpenter - 1982 - Limits on the activation parameters for automeriza.pdf;/Users/monino/Zotero/storage/WRSENMYS/ja00387a065.html},
  journal = {J. Am. Chem. Soc.},
  number = {23}
}