Easy input

In this chapter, we will introduce the BDF easy input control keyword.

Easy Input Keywords

### Required Parameters: Method/Functional/Basis, Functional/Basis, Method/Basis

Required parameters in easy input mode are used to set computational methods, functionals for DFT/TDDFT calculations, basis sets, etc. Currently supported computational methods include:

Method	Functionality
HF	Hatree-Fock
RHF	Restricted Hatree-Fock
UHF	Unrestricted Hartree-Fock
ROHF	Restricted open-shell Hatree-Fock
KS	Kohn-Sham
RKS	Restricted Kohn-Sham
UKS	Unrestricted Kohn-Sham
TDDFT	Time-dependent density functional theory
TDA	Tamm-Dancoff Approximation
X-TDDFT	Extended spin-adapted TDDFT
X-TDA	Extended spin-adapted TDA
TDDFT-SOC	TDDFT with spin-orbit coupling
TDA-SOC	TDA with spin-orbit coupling
X-TDDFT-SOC	Extended spin-adapted TDDFT with spin-orbit coupling
X-TDA-SOC	Extended TDA with SOC
TDDFT-NAC	TDDFT with non-adabatic coupling
TDA-NAC	TDA with non-adabatic coupling
X-TDDFT-NAC	X-TDDFT with non-adabatic coupling
X-TDA-NAC	X-TDA with non-adabatic coupling
MP2	Mollor-Plesset second order perturbation theory
RI-MP2	MP2 using Resolution of Identity

Hamiltonian and Spin-Orbit Coupling

### hamilton Parameter Type: String, Optional

Set the relativistic Hamiltonian for calculation

Default: nonrel (uses sf-X2C when relativistic basis sets are employed)

Options: sf-X2C, sf-X2C-AXR, sf-X2C-AU

### SOC Parameter Type: Bool, Optional

Request spin-orbit coupling (SOC) calculation and set corresponding SOC operator. If method is TDDFT, performs SOC calculation based on TDDFT; if method is TDA, performs SOC calculation based on TDA.

Default: DKH1e+mf1c

Options: DKH1e+mf1c, DKH1e, BP; DKH1e+mf1c for all-electron calculations, BP operator for relativistic effective potentials.

Note

Default Principle: If Hamiltonian is specified, BDF will select appropriate Hamiltonian based on basis functions. For all-electron basis sets considering relativistic effects or non-relativistic all-electron basis sets, scalar terms use sf-X2C Hamiltonian, spin-orbit coupling operator uses DKH1e+mf1c. Users can set to DKH1e, but this may introduce significant errors for light elements. For relativistic effective potentials and basis sets, the potential already includes relativistic effects, so no Hamiltonian needs to be set; SOC operator defaults to BP.
Users input TDDFT/functional/basis SOC (using SOC keyword) is equivalent to setting X-TDDFT/functional/basis, with Hamiltonian and SOC operators set according to default principles.

Coordinate Units, Charge, and Spin Multiplicity

### unit Parameter Type: String, Optional

Atomic coordinate units

Default: angstrom

Options: angstrom, Bohr

### spinmulti Parameter Type: Integer, Optional

Spin multiplicity, 2S+1

Default: 1 for even-electron systems, 2 for odd-electron systems

### charge Parameter Type: Integer, Optional

Charge number

Default: 0

Spin-Adapted TDDFT and TDA

### SpinAdapt

Set spin-adapted TDDFT or TDA. TDDFT/functional/basis SpinAdapt is equivalent to X-TDDFT/functional/basis or X-TDA. Only meaningful for open-shell systems.

Non-Adiabatic Coupling

### NAC Parameter Type: Bool, Optional

Non-adiabatic coupling (NAC) calculation based on time-dependent density functional theory (TDDFT)

Default: False

Potential Energy Surfaces and Structural Optimization

### opt Parameter Type: Bool, Optional

Molecular geometry optimization for stable points.

Default: False

### opt+freq Parameter Type: Bool, Optional

Molecular geometry optimization for stable points, followed by frequency calculation.

Default: False

### ts+freq Parameter Type: Bool, Optional

Transition state optimization, followed by frequency calculation.

Default: False

### freq Parameter Type: Bool, Optional

Frequency calculation.

Default: False

### scan Parameter Type: Bool, Optional

Molecular potential energy surface scan, requires internal coordinate input.

Default: False

### scan+opt Parameter Type: Bool, Optional

Flexible molecular potential energy surface scan (optimizes other coordinate parameters while fixing certain internal coordinate parameters), requires internal coordinate input.

Default: False

Acceleration Algorithms

### MPEC+COSX Parameter Type: Bool, Optional

Accelerate SCF, TDDFT energy and gradient calculations using Multipole Expansion of Coulomb Potential (MPEC) and Chain-Of-Spheres Exchange (COSX).

Default: False

### RI Parameter Type: Bool, Optional

Accelerate SCF, TDDFT or MP2 calculations using Resolution of Identity (RI), requires auxiliary basis sets.

Default: False

Tip

RI in BDF is mainly used to accelerate MP2 calculations. For SCF and TDDFT, MPEC+COSX is preferred. This method is unique to BDF, offers comparable accuracy to RI, and does not require auxiliary basis sets.