How to find the name of a Nix package to install it in configuration.nix?

Suppose I search for a package to install using [nix-env](https://nixos.org/nix/manual/#sec-nix-env) 's [--query](https://nixos.org/nix/manual/#idm46912466717184) operation: $ nix-env -qa 'aspell.*en' aspell-dict-en-7.1-0 I write this package name in /etc/nixos/configuration.nix, NixOS's main [configuration file](http://nixos.org/nixos/manual/index.html#ch-configuration) : environment.systemPackages = with pkgs; [ aspell-dict-en ]; Yet if I run sudo nixos-rebuild switch, NixOS command to [update the configuration](http://nixos.org/nixos/manual/index.html#sec-changing-config) and install all system-wide packages specified by [declaratively](http://nixos.org/nixos/manual/index.html#sec-declarative-package-mgmt) , it aborts with an error: error: undefined variable ‘aspell-dict-en’ at /etc/nixos/configuration.nix:44:5 I know that for many packages, although not all, the name that nix-env returns and the name one should specify in environment.systemPackages configuration option are different, but I don't understand the logic. How do I install a package that I found through nix-env?