Add optional DFTB+ external backend

README.md

@@ -21,6 +21,22 @@ Open Quantum Platform ([OpenQP](https://pubs.acs.org/doi/10.1021/acs.jctc.4c0111
 - [OpenTrustRegion library](https://github.com/eriksen-lab/opentrustregion) for stable SCF convergence
 - Native PySCF-based advanced initial guesses, plus optional [MOKIT](https://github.com/1234zou/MOKIT) support for broader external wavefunction conversion workflows
 
+### DFTB+ External Backend
+
+This branch includes an optional external DFTB+ backend for ground-state workflows. It shells out to a user-provided `dftb+` executable and Slater-Koster parameter directory configured in the `[dftb]` input section.
+
+| Capability | Status | Scope |
+| --- | --- | --- |
+| Energy | Supported | Parses DFTB+ `results.tag` or `detailed.out`. |
+| Gradient | Supported | Parses DFTB+ forces and returns OpenQP gradients. |
+| Geometry optimization | Supported | Ground-state `runtype=optimize`, `lib=scipy`, `istate=0`, using DFTB+ energy/gradient callbacks. |
+| TD-DFTB excited states | Unsupported | DFTB+ excited-state outputs are not parsed or mapped into OpenQP TD data. |
+| Nonadiabatic couplings | Unsupported | OpenQP NAC workflows require TDHF/MRSF state data unavailable from this bridge. |
+| Spin-flip DFTB | Unsupported | OpenQP spin-flip response is native TDHF/MRSF only. |
+| Hessian/frequencies | Unsupported | No DFTB+ Hessian parser or numerical-Hessian callback is wired. |
+| Molecular dynamics | Unsupported | No OpenQP DFTB+ MD workflow is wired in this branch. |
+| Native OpenQP DFTB Hamiltonian | Unsupported | This backend is external DFTB+ only. |
+
 ### Upcoming Features
 - **Efficient electrostatic embedding QM/MM** by [ESPF QM/MM](https://doi.org/10.1063/5.0133646)
 - **Spin-Orbit Coupling** by [**Relativistic** MRSF-TDDFT](https://doi.org/10.1021/acs.jctc.2c01036)
@@ -153,4 +169,3 @@ If you use OpenQP in your research, please cite the following papers:
 ### Legal Notice
 
 See the separate LICENSE file.
-

examples/DFTB/H2O_DFTBPLUS_GRADIENT.inp

@@ -0,0 +1,21 @@
+# H2O DFTB+ single-point gradient example.
+#
+# This input requires an external DFTB+ installation and compatible
+# Slater-Koster parameter directory. Set [dftb] executable/sk_path for your
+# local installation before running; CI fixture tests do not require DFTB+.
+
+[input]
+method=dftb
+runtype=grad
+charge=0
+system=
+  O  0.000000  0.000000  0.000000
+  H  0.000000  0.757160  0.586260
+  H  0.000000 -0.757160  0.586260
+
+[dftb]
+executable=dftb+
+sk_path=/path/to/slater-koster/3ob-3-1
+scc=True
+max_scc_iterations=100
+timeout=300

examples/DFTB/H2O_DFTBPLUS_OPTIMIZE.inp

@@ -0,0 +1,27 @@
+# H2O DFTB+ ground-state geometry optimization example.
+#
+# This input requires an external DFTB+ installation and compatible
+# Slater-Koster parameter directory. Set [dftb] executable/sk_path for your
+# local installation before running; CI fixture tests do not require DFTB+.
+
+[input]
+method=dftb
+runtype=optimize
+charge=0
+system=
+  O  0.000000  0.000000  0.000000
+  H  0.000000  0.757160  0.586260
+  H  0.000000 -0.757160  0.586260
+
+[dftb]
+executable=dftb+
+sk_path=/path/to/slater-koster/3ob-3-1
+scc=True
+max_scc_iterations=100
+timeout=300
+
+[optimize]
+lib=scipy
+optimizer=bfgs
+istate=0
+maxit=50