add template flake for use with nix flake init

Author: sheepforce

README.md

@@ -27,6 +27,36 @@ Nix package manager together with NixOS-QChem (commit <10 digits of SHA-1>) [1].
 
 ## Usage
 
+### Template Flake
+We provide a template [Nix Flake](https://nixos.wiki/wiki/Flakes) ready for use and with reasonable defaults and easy customisability.
+Get the template:
+
+```bash
+nix flake init -t github:nix-qchem/nixos-qchem
+```
+
+In the so obtained `flake.nix` look for `EDITME` tokens to find places for common modifications such as:
+
+  * CPU architecture specific optimisations
+  * Enabling or disabling certain unfree packages
+  * Definition of a custom Python environment with packages you require
+  * ...
+
+A useful directory layout for a project may look like this:
+
+```
+PROJECT/
+├── calculations/
+├── .envrc
+└── nix
+    ├── flake.lock
+    └── flake.nix
+```
+
+With `.envrc` containing `use flake ./nix`.
+If you are using [DirEnv](https://direnv.net/) you get your software environment for your project by merely entering the `PROJECT` directory.
+
+
 ### Overlay
 The repository comes as a nixpkgs overlay (see [Nixpkgs manual](https://nixos.org/nixpkgs/manual/#chap-overlays) for how to install an overlay).
 The contents of the overlay will be placed in an attribute set under nixpkgs (default `qchem`). The original, but overridden nixpkgs will be placed in `qchem.pkgs`. This allows for composition of the overlay with different variants.

flake.nix

@@ -83,5 +83,20 @@
         pythonQchem = import ./pythonPackages.nix pkgs.config.qchem-config.prefix pkgs.config.qchem-config pkgs nixpkgs;
         default = self.overlays.qchem;
       };
+
+      templates.default = {
+        path = ./template;
+        description = "NixOS-QChem template flake with reasonable defaults and editable configuration";
+        welcomeText = ''
+          Welcome to the NixOS-QChem template flake with optimised packages for computational chemistry!
+
+          If you use this flake and/or packages from NixOS-QChem, please cite https://doi.org/10.1002/qua.26872 in your work!
+
+          The flake aims for reasonable defaults but can be customised to your needs.
+          Look for "EDITME" tokens in the flake.nix to find places where you can customise the flake,
+          e.g. by adding or removing unfree packages, changing CPU architecture specific optimisations,
+          or constructing a Python environment suiting your requirements.
+        '';
+      };
     };
 }

template/flake.nix

@@ -0,0 +1,125 @@
+{
+  description = "A flake for computational chemistry, based on NixOS-QChem";
+
+  inputs = {
+    #qchem.url = "github:nix-qchem/nixos-qchem";
+    qchem.url = "../";
+  };
+
+  nixConfig = {
+    extra-substituters = [ "https://nix-qchem.cachix.org" ];
+
+    allow-import-from-derivation = "true";
+  };
+
+  outputs = { self, qchem }:
+    let
+      # EDITME
+      # Overlay to enable Gaussian support. Uncomment below in `pkgs` to enable.
+      enableGaussian = final: prev: {
+        qchem = prev.qchem // {
+          gaussian = prev.callPackage "${qchem}/pkgs/apps/gaussian" {
+            # Set a version
+            version = "16c02";
+
+            # Either set optpath
+            optpath = "/opt";
+            # or g16Root and g16Dir
+            #g16Root = "/cluster/apps/gaussian/g16_c02";
+            #g16Dir = "/cluster/apps/gaussian/g16_c02/g16";
+          };
+        };
+      };
+
+      # EDITME
+      # Disables unfree packages by default to avoid non-building packages, that
+      # use unfree ones as dependencies, e.g. Pysisyphus or SHARC.
+      # Comment out lines to enable the respective package.
+      filterUnfree = final: prev: {
+        qchem = prev.qchem // {
+          turbomole = null;
+          cefine = null;
+          cfour = null;
+          gamess-us = null;
+          gaussview = null;
+          mrcc = null;
+          orca = null;
+          vmd = null;
+          mesa-qc = null;
+          mcdth = null;
+          nixGL = null;
+        };
+      };
+
+      # EDITME
+      # Python environment for computational chemistry. Add or remove packages
+      # as needed.
+      pyCompEnv = pkgs.qchem.python3.withPackages (ps: with ps; [
+        jupyterlab
+        numpy
+        scipy
+        scikit-learn
+        ase
+        pyscf
+        psi4
+      ]);
+
+      system = "x86_64-linux";
+
+      pkgs = import qchem.inputs.nixpkgs {
+        inherit system;
+
+        # EDITME
+        overlays = [
+          qchem.overlays.default
+          filterUnfree
+          # enableGaussian # Uncomment to enable Gaussian support
+        ];
+
+        config = {
+          allowUnfree = true;
+
+          # EDITME
+          qchem-config = {
+            # Set to a GCC architecture to optimise packages for.
+            # See https://gcc.gnu.org/onlinedocs/gcc/x86-Options.html for options.
+            optArch = "haswell";
+
+            # Set to an installation path of Gaussian/MatLab outside the Nix
+            # store if you have one. Should have a directory structure like this:
+            #   "${optpath}/gaussian/g16/"
+            #   "${optpath}/matlab-R2024/"
+            #optpath = "/opt";
+
+            # Set and uncomment if you have a MolPro license key.
+            #licMolpro = "XXXX-XXXX-XXXX-XXXX";
+
+            # Set and uncomment if you have a Q-Chem license key file.
+            #licQChem = "/path/to/qchem.lic";
+
+            # Uncomment if you have a server from which non-redistributable
+            # software can be downloaded
+            #srcurl = "http://myserver/nix-src/";
+          };
+        };
+      };
+    in
+    {
+      # EDITME
+      # A shell to obtain via `nix develop` or DirEnv, useful to run
+      # programs interactively. Contains a Python environment for computational
+      # chemistry. See above. Add/remove packages as needed.
+      devShells."${system}".default = with pkgs; mkShell {
+        buildInputs = [
+          pyCompEnv
+          pkgs.qchem.xtb
+        ];
+      };
+
+      packages."${system}" = with pkgs.lib.attrsets;
+        filterAttrs (_: val: isDerivation val)
+          (pkgs.qchem // {
+            inherit (pkgs) lammps lammps-mpi tachyon;
+          });
+    };
+}