add nix build instructions and README section (#173)

# Conflicts:
#	README.md

README.md

@@ -38,6 +38,7 @@ single- and/or multi-reference wave functions:
     * [Conda](#conda)
     * [Spack](#spack)
     * [Guix](#guix)
+    * [Nix](#nix)
     * [Debian/Ubuntu](#debianubuntu)
   * [Installation from source](#installation-from-source)
     * [Minimal requirements (for users):](#minimal-requirements-for-users)
@@ -100,6 +101,23 @@ It can be installed as follows:
 guix package --cores=`getconf _NPROCESSORS_ONLN` --install-from-file=trexio.scm
 ```
 
+#### Nix
+
+The official releases of TREXIO `>=2.5.` can be used and installed via [Nix](https://nixos.org/).
+This repository provides a [Nix Flake](https://nixos.wiki/wiki/Flakes), where [tools/nix/trexio.nix](https://github.com/TREX-CoE/trexio/blob/master/tools/nix/trexio.nix) provides the build specification.
+You can inspect the flake or build the package with
+
+```
+nix flake show github:TREX-CoE/trexio
+nix build github:TREX-CoE/trexio
+```
+
+TREXIO is also part of [NixPkgs](https://github.com/NixOS/nixpkgs) and can be used via that channel.
+
+```
+nix build nixpkgs#trexio
+```
+
 #### Debian/Ubuntu
 
 The official release of TREXIO `2.2.0` is available as a Debian (`.deb`) package thanks to the [Debichem Team](https://wiki.debian.org/Debichem).

flake.lock

@@ -0,0 +1,60 @@
+{
+  "nodes": {
+    "flake-utils": {
+      "inputs": {
+        "systems": "systems"
+      },
+      "locked": {
+        "lastModified": 1731533236,
+        "narHash": "sha256-l0KFg5HjrsfsO/JpG+r7fRrqm12kzFHyUHqHCVpMMbI=",
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "rev": "11707dc2f618dd54ca8739b309ec4fc024de578b",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "type": "github"
+      }
+    },
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1733852629,
+        "narHash": "sha256-lXMmrcEg1WxEdPiXpUfkxbJij0xS5acao1Jz4lnrNPY=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "9ad61cfc19c1f66e7d7d1ac9c71ae1b187065580",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "flake-utils": "flake-utils",
+        "nixpkgs": "nixpkgs"
+      }
+    },
+    "systems": {
+      "locked": {
+        "lastModified": 1681028828,
+        "narHash": "sha256-Vy1rq5AaRuLzOxct8nz4T6wlgyUR7zLU309k9mBC768=",
+        "owner": "nix-systems",
+        "repo": "default",
+        "rev": "da67096a3b9bf56a91d16901293e51ba5b49a27e",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-systems",
+        "repo": "default",
+        "type": "github"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}

flake.nix

@@ -0,0 +1,24 @@
+{
+  description = "TREX I/O library";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs";
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs = { self, nixpkgs, flake-utils }:
+    let overlay = import ./tools/nix/overlay.nix;
+    in flake-utils.lib.eachDefaultSystem (system:
+      let
+        pkgs = import nixpkgs {
+          inherit system;
+          overlays = [ overlay ];
+        };
+      in
+      {
+        packages.default = pkgs.trexio;
+      }
+    ) // {
+      overlays.default = overlay;
+    };
+}

tools/nix/overlay.nix

@@ -0,0 +1,11 @@
+final: prev: {
+  trexio = final.callPackage ./trexio.nix { };
+
+  haskell = prev.haskell // {
+    packageOverrides = hfinal: hprev: {
+      trexio-hs = hfinal.callCabal2nix "trexio" ../haskell {
+        inherit (final) trexio;
+      };
+    };
+  };
+}

tools/nix/trexio.nix

@@ -0,0 +1,55 @@
+{ stdenv
+, lib
+, fetchFromGitHub
+, cmake
+, gfortran
+, hdf5
+, python3
+, emacs
+, swig
+}:
+
+stdenv.mkDerivation rec {
+  pname = "trexio";
+  version = "2.5.0";
+
+  src = lib.cleanSourceWith {
+    src = ../../.;
+    filter = path: type: ! (builtins.elem (builtins.baseNameOf path) [
+      "haskell"
+      "docker"
+      "helpers-debian"
+      "ocaml"
+      "rust"
+      "python"
+    ] && type == "directory" );
+  };
+
+  postPatch = ''
+    patchShebangs tools/*
+  '';
+
+  nativeBuildInputs = [
+    cmake
+    gfortran
+    emacs
+    swig
+    python3
+  ];
+
+  buildInputs = [
+    hdf5
+  ];
+
+  outputs = [ "out" "dev" ];
+
+  doCheck = true;
+
+  meta = with lib; {
+    description = "File format and library for the storage of quantum chemical wave functions";
+    homepage = "https://trex-coe.github.io/trexio/";
+    downloadPage = "https://github.com/TREX-CoE/trexio";
+    license = licenses.bsd3;
+    maintainers = [ maintainers.sheepforce ];
+  };
+}

version_memo.txt

@@ -5,4 +5,4 @@ To update the version, change:
 - ocaml/trexio/trexio.opam
 - python/pytrexio/_version.py
 - rust/trexio/Cargo.toml
-
+- tools/nix/trexio.nix