# ORCA 6 Changes

## Highlights

### SCF and Single Reference 

-   LeanSCF: reduced memory, more robust convergence
-   Electric field optimizations
-   General ROHF implementation (SCF/Gradient) with all approximations
-   General CSF ROHF
-   New density functionals
-   Delta-SCF
-   UHF STEOM-CCSD
-   UHF-IP-EOM-CCSD
-   UHF-EA-EOM-CCSD
-   Regularized OOMP2
-   Solvation in OOMP2
-   Improved stability analysis featuring all approximations, solvation etc
-   MixGuess to converge to biradicaloid open shell singlet type broken symmetry solutions
-   Approximate Spin Projection Method for broken symmetry calculations (SCF/Gradient)

### Multi Reference 

-   TRAH, AVAS, MCRPA
-   Linear response CASSCF
-   Vastly improved Recursive CI coupling coefficient generation

### Automatic Code generation

-   MPn
-   CCSD(T) gradients
-   CCSDT

### Relativity 

-   X2C
-   New and consistent DKH infrastructure

### Solvation & Embedding 

-   DLPNO-CCSD(T) PTES Approach
-   SMD analytical Hessian
-   Dynamically adjusted radii: DRACO
-   Improved surface grids
-   Interface to openCOSMO-RS
-   Explicit Solvator
-   Molecule Docker
-   FMM implementation for embedding
-   CIM implementation works with DLPNO-CC, DLPNO-MP2

### Optimization 

-   More robust optimizer (fewer cycles, fewer cases with negative frequencies)
-   GOAT global optimizer and conformer generator
-   Basis set limit extrapolated optimizations through compound scripts
-   Extrapolation with counterpoise correction through compound scripts

### Hessian 

-   Group parallelization
-   Performance improvements

### Excited States

-   Analytical gradient for meta-GGA functionals

### New Spectroscopic Properties 

-   VCD implementation at the SCF level
-   MCD with vibronic structure
-   General spin ROCIS
-   Higher order moments and exact field matter coupling
-   Spin rotation constants

### Misc Properties 

-   Local dipole moments and polarizabilities
-   Frequency dependent electric properties
-   VPT2 enhancements
-   Restructured NMR simulation program `orca_nmrspectrum`
-   MBIS charges

### Workflow & Interfacing 

-   Property file: Machine readable, Human readable summary of ORCA run
-   Compound: vastly improved Syntax, features, optimizations, ...
-   `orca_2json`: generate integrals, property file, run backwards to get MOs into ORCA
-   Citation tool for helping find the right references


## Other Changes

### SCF and Infrastructure

- Significant improvements to the SOSCF solver to make it more robust, preventing huge steps that break the SCF. Overall improvements on the DIIS solvers.
- Due to the SCF updates, the AutoTRAH is now not so often needed and will start now only from above 50 cycles (`AutoTRAHIter`).
- Improvements to the memory handling of TD-DFT, CP-SCF and the Hessian

### Basis sets

- def-TZVP and ma-def-TZVP pseudo-potential basis sets for the actinides (Z = 89, Ac - 103, Lr)
- Lehtola's hydrogenic gaussian basis set family (HGBS) including polarized (HGBSP) and augmented (AHGBS, AHGBSP) variants for all elements up to Oganesson (Z = 118)
- def2-SVPD, def2-TZVPD, def2-TZVPPD, def2-QZVPD, def2-QZVPPD basis sets for lanthanoids
- `!MINIX` now correctly activates the corresponding ECP
- Added user-specified L-limit to AutoAux `AutoAuxLLimit`
- Fixed segfault in dhf-ECP
- Fix for `DelECP` in `%coords`
- Added `ReadFragBasis` keywords read fragment-specific basis sets from a file

### Solvation

- New charge correction / compensation algorithm (corrected charges printed in an additional file)
- C-PCM/B scheme for QM/MM calculations
- DDCOSMO and CPCM/X available for XTB calculations and QM/MM calculations
- Generalization of names within all solvation models (C-PCM/SMD/ALPB/DDCOSMO/CPCM-X)
- New discretization scheme for the cavity (C-PCM) based on a constant number of charges per unit of area
 
### DFT

- Allow LibXC functional customization via external parameters
- Simple input keywords added for some LibXC functionals
- Added wB97M(2) functional parameters: must be used with wB97M-V orbitals in a two-step job (compound script available)
- Bugfixes for LibXC combined `*_xc_*` functionals
- Fixed crash for D4 + ghost atoms

### Excited states

- Analytical gradient for meta-GGA functionals
- Small bugfix to spin-adapted triplets and NACMEs.
- The FolllowIRoot for excited state optimization uses now a much more robust algorithm.

### Relativity

- Enabled `NumGrad` with relativistic methods
- Second order DKH picture-change correction of contact density
- Minor fixes in DKH picture-change corrections of magnetic properties
- Picture change corrections are activated automatically

### Multiscale

- Reading PDB files for 10k+ atoms with HETATMs now possible
- Enabled correct FlipSpin behavior with QMMM
- More efficient MM Module
- Implemented wall potential

### Coupled cluster / DLPNO

- Implemented energy ordering for PNO generation
- Added semicore treatment for DLPNO
- Enable DLPNO-CCSD(T) calculations to run DLPNO-CCSD unrelaxed densities

### MP2
- Corrected memory estimates and batching in response and gradient
- Removed the slow and limited analytic (RI-)MP2 Hessian code
- Removed non-default Gamma-in-core option for RI-MP2 response
- Disabled single-precision calculations
- Disabled SemiDirect option in AO-MP2
- Enabled range-separated DHDFT gradients with RIJDX

### NEB

- Improved IDPP initial path
- More efficient GFN-xTB runs for NEB

### COSX

- Improvements to numerical integration grids, both for DFT and COSX
- Faster grid step
- Improved performance and accuracy in COSX, also for the gradient and Hessian

### Properties

- NMR spin-spin coupling: 
  - Added `SpinSpinElemPairs` and `SpinSpinAtomPairs` keywords to limit which couplings are computed
  - Reduced the number of CP-SCF perturbations necessary via a stochastic selection 
  - DSO term was transposed.
  - Off-diagonal PSO elements had the wrong sign
  - Efficiency improvement: solve SD/FC CP-SCF equations in restricted mode for RHF, instead of always using UHF
- Optimized numeric integration for HFC gauge correction
- Removed `RITRAFO` option for CP-SCF
- Switched to `tau=Dobson` as default handling of the kinetic energy density in meta-GGA magnetic properties with GIAOs

### Hessian

- Improvements to the Hessian to avoid accumulation on numerical noise and reduce the number of spurious negative frequencies.

### Geometry Optimization

- Several improvements to the geometry optimization, making is much more stable. Complete redesign of the Cartesian optimizer (!COPT), making it quick enough to be used together with faster methods.
- Fallbacks in the geometry optimization in case something fails, e.g. if the internal coordinates are unacceptable.
- Arbitrary spherical, ellipsoidal or box-like wall potentials can be added, which will reflect on the energy and gradients and can be used during geometry optimization.

### Miscellaneous

- CHELPG charges that reproduce the ESP together with the molecular dipole moment
- Fixed issues with constraints in multi-step jobs
- Molden output: store ECP info in `[Pseudo]` block, set point charge atomic number to 0, handling of ghost atoms
- Made the `ExtOpt` interface easier to use
- Store energy from NEB and IRC in the XYZ file