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## Table of contents
[[_TOC_]]
## Details
The `QM` section handles the reference and ChronusQ job type specification. **The `QM` section is a required section for all ChronusQ jobs.**
## Keywords
| Keyword | Type | Description | Default | Required |
| ------- | ---- | ----------- | ------- | -------- |
| `REFERENCE` | String | Type of reference wavefunction | N/A | Yes |
| `NUCREFERENCE`| String | Type of NEO reference wavefunction | N/A | No|
| `JOB` | String | Type of calculation | N/A | Yes |
| `X2CTYPE` | String | Type of X2C transformation | `DEFAULT` | No |
| `ATOMICX2C` | String | Type of Atomic X2C transformation | `OFF` | No |
| `SPINORBITSCALING` | String | Type of spin-orbit scaling technique | `DEFAULT` | No |
### The `REFERENCE` Keyword
The `QM.REFERENCE` keyword allows for specification of the reference wave function for SCF and post-SCF ChronusQ calculations. ChronusQ currently supports Hartree-Fock and Kohn-Sham reference choices. The `REFERENCE` keyword may be constructed systematically as
```
REFERENCE = [ REAL/COMPLEX ] <R/RO/U/G/2C><HF/FUNCTIONAL>
```
The `REAL/COMPLEX` specification is optional, and a canonical choice will be chosen for the user should it not be specified (`COMPLEX` for 2-Component / GIAO references, `REAL` otherwise). The second two keyword fields in the above template are required, and may be combined freely.
`R/RO/U/G/X2C` specifies the desired spin symmetry of the reference wave function: Restricted (S<sup>2</sup> eigenfunction, even number of particles), Restricted Open (S<sup>2</sup> eigenfunction, any number of particles), Unrestricted (S<sub>z</sub> eigenfunction), Generalized (2-Component, no spin symmetry), X2C (deprecated, "Exact" 2-Component Relativistic, with one-body spin-orbit coupling, no spin symmetry), respectively.
`HF` will generate a Hartree-Fock wave function, and `FUNCTIONAL` is a place holder to specify a Kohn-Sham wave function with the `FUNCTIONAL` DFT functional. (See below) **`RO` is currently only compatible with `HF`, not Kohn-Sham.**
#### DFT Functionals
ChronusQ supports the following DFT functionals (10/9/2020):
| Name | Type | Description |
| ---- | ---- | ----------- |
| `SLATER` | Pure LDA Exchange | Slater exchange |
| `LSDA`/`LDA` | Pure LDA Exchange/Correlation | Slater exchange + VWN3 correlation |
| `SVWN5` | Pure LDA Exchange/Correlation | Slater exchange + VWN5 correlation |
| `B88` | Pure GGA Exchange | B88 exchange |
| `BLYP` | Pure GGA Exchange/Correlation | B88 exchange + LYP correlation |
| `PBEXPBEC` | Pure GGA Exchange/Correlation | PBE exchange + PBE correlation |
| `B3LYP` | Hybrid GGA Exchange/Correlation | B3LYP functional |
| `B3PW91` | Hybrid GGA Exchange/Correlation | B3PW91 functional |
| `PBE0` | Hybrid GGA Exchange/Correlation | PBE0 functional |
| `BHANDHLYP` | Hybrid GGA Exchange/Correlation | BHandHLYP functional |
| `BHANDH` | Hybrid GGA Exchange/Correlation | BHandH functional |
Addition of more functionals is possible - if a specific functional is required for your research, please [open an issue!](https://urania.chem.washington.edu/chronusq/chronusq_public/-/issues)
### The `NUCREFERENCE` Keyword
The `QM.REFERENCE` keyword allows for specification of the reference wave function for quantum nuclei in the SCF and post-SCF ChronusQ procedures. Currently both NEO-HF and NEO-DFT wavefunction options are available. It is assumed that systems occupy high-spin configurations, so by convention unrestricted wavefunctions should be used. Note that Generalized and X2C options are not currently implemented for NEO methods.
REAL/COMPLEX specification is optional as with `QM.REFERENCE`, but NEO wavefunctions must match the type of the electronic reference (only `REAL/REAL` or `COMPLEX/COMPLEX` for `REF/NUCREF` are allowed).
`HF` will generate a respective NEO-HF wavefunction, while quantum-nuclear functionals have the same conventions as their electronic counterparts. Currently implemented functionals are listed below.
### NEO-DFT Functionals
| Name | Type | Description |
|----- | ---- | ----------- |
| epc17-2 | NEO-LDA-like Functional | Quantum electron-proton correlation functional |
| epc19 | NEO-GGA-like Functional | Quantum electron-proton correlation with density gradient included |
### The `JOB` Keyword
The `JOB` keyword controls the type of calculation done by ChronusQ. The currently supported job types are:
| Value | Description |
| ----- | ----------- |
| `SCF` | Self-consistent orbital optimization |
| `RESP` | Response theory |
| `RT` | Electron dynamics |
For more information, see the respective input sections of each.
### The `X2CTYPE` Keyword
When the reference is specified as `X2C`, different methodologies can be used to define the transformation between the four- and two-component Hamiltonians. The `X2CTYPE` keyword controls the type of transform used.
| Value | Description |
| ----- | ----------- |
| `OFF` | Turns off X2C |
| `ONEELECTRON`/`ONEE` | The standard core-Hamiltonian X2C transformation algorithm |
| `SPINFREE` | One-electron X2C, but only include scalar relativistic effect |
| `FOCK` | X2C transformation based on the four-component Fock matrix |
| `DEFAULT` | `ONEE` in case of `x2c` reference, `OFF` otherwise. |
### The `ATOMICX2C` Keyword
When the `X2CTYPE` keyword is specified as `SPINFREE` or `ONEE`, either a standard molecular X2C transformation is performed, or local approximations can be utilized. The following options control the types of local approximation.
| Value | Description |
| ----- | ----------- |
| `OFF` | The standard X2C transformation algorithm without local approximation |
| `ALH` | Atomic local approximation to the Hamiltonian |
| `ALU` | Atomic local approximation to the transformation matrix |
| `DLH` | Diagonal local approximation to the Hamiltonian |
| `DLU` | Diagonal local approximation to the transformation matrix |
### The `SPINORBITSCALING` Keyword
When the reference is specified as `X2C`, different methodologies can be used to artificially include the two-electron spin-orbit effect. The `SPINORBITSCALING` keyword switches such scaling.
| Value | Description |
| ----- | ----------- |
| `NOSCALING` | Turns off two-electron spin-orbit scaling |
| `BOETTGER` | The Boettger factor scaling technique |
| `ATOMICMEANFIELD`/`AMFI` | Atomic mean-field two-electron spin-orbit effect |
| `DEFAULT` | `BOETTGER` in case of one-electron spin-orbit X2C calculation, `NOSCALING` otherwise. |
## PROTQM Section
For NEO calculations, a second section needs to be included that designates the PROTQM options. Since `JOB` is specified previously, only reference information needs to be included. The same conventions mentioned previously apply here. GIAO and relativistic options are not yet implemented for nuclear wavefunctions. Below is the list of keywords needed for this section.
| Keyword | Type | Description | Default | Required |
| ------- | ---- | ----------- | ------- | -------- |
| `REFERENCE` | String | Type of reference wavefunction | N/A | Yes |
## Examples
### Restricted Hartree-Fock Energy
```
[QM]
Reference = RHF
Job = SCF
```
### Unrestricted Kohn-Sham (B3LYP) Response
```
[QM]
Reference = UB3LYP
Job = Resp
```
### Complex Generalized Kohn-Sham (LSDA) Energy
```
[QM]
Reference = Complex GLSDA
Job = SCF
```
### Relativistic Kohn-Sham (BHandH) Electron Dynamics
```
[QM]
Reference = X2CBHandH
Job = RT
```
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