Q-Chem 5.1 User’s Manual

7.1 General Excited-State Features

As for ground state calculations, performing an adequate excited-state calculation involves making an appropriate choice of method and basis set. The development of effective approaches to modeling electronic excited states has historically lagged behind advances in treating the ground state. In part this is because of the much greater diversity in the character of the wave functions for excited states, making it more difficult to develop broadly applicable methods without molecule-specific or even state-specific specification of the form of the wave function. Recently, however, a hierarchy of single-reference ab initio methods has begun to emerge for the treatment of excited states. Broadly speaking, Q-Chem contains methods that are capable of giving qualitative agreement, and in many cases quantitative agreement with experiment for lower optically allowed states. The situation is less satisfactory for states that involve two-electron excitations, although even here reasonable results can sometimes be obtained. Moreover, some of the excited state methods can treat open-shell wave functions, e.g. diradicals, ionized and electron attachment states and more.[Krylov(2017)]

In excited-state calculations, as for ground state calculations, the user must strike a compromise between cost and accuracy. This chapter summarizes Q-Chem’s capabilities in four general classes of excited state methods:

Note: Core electrons are frozen by default in most correlated excited-state calculations (see Section 6.2).

In general, a basis set appropriate for a ground state density functional theory or a Hartree-Fock calculation will be appropriate for describing valence excited states. However, many excited states involve significant contributions from diffuse Rydberg orbitals, and, therefore, it is often advisable to use basis sets that include additional diffuse functions. The 6-31+G* basis set is a reasonable compromise for the low-lying valence excited states of many organic molecules. To describe true Rydberg excited states, Q-Chem allows the user to add two or more sets of diffuse functions (see Chapter 8). For example the 6-311(2+)G* basis includes two sets of diffuse functions on heavy atoms and is generally adequate for description of both valence and Rydberg excited states.

Q-Chem supports four main types of excited state calculation:

Note: EOM-CC and most of the CI codes are part of CCMAN and CCMAN2. CCMAN is a legacy code which is being phased out. All new developments and performance-enhancing features are implemented in CCMAN2.

METHOD

Specifies the level of theory.


TYPE:

STRING


DEFAULT:

None

No Correlation


OPTIONS:

CIS

Section 7.2.1

CIS(D)

Section 7.6.1

RI-CIS(D)

Section 7.6.2

SOS-CIS(D)

Section 7.6.3

SOS-CIS(D0)

Section 7.6.4

CISD

Section 7.7.2

CISDT

Section 7.7.2

EOM-OD

Section 7.7.2

EOM-CCSD

Section 7.7.2

EOM-MP2

Section 7.7.9

EOM-MP2T

Section 7.7.9

EOM-CCSD-S(D)

Section 7.7.10

EOM-MP2-S(D)

Section 7.7.10

EOM-CCSD(dT)

Section 7.7.21

EOM-CCSD(fT)

Section 7.7.21

EOM-CC(2,3)

Section 7.7.18

ADC(0)

Section 7.8

ADC(1)

Section 7.8

ADC(2)

Section 7.8

ADC(2)-X

Section 7.8

ADC(3)

Section 7.8

SOS-ADC(2)

Section 7.8

SOS-ADC(2)-X

Section 7.8

CVS-ADC(1)

Section 7.8

CVS-ADC(2)

Section 7.8

CVS-ADC(2)-X

Section 7.8

CVS-ADC(3)

Section 7.8

RAS-CI

Section 7.9

RAS-CI-2

Section 7.9


RECOMMENDATION:

Consult the literature for guidance.