diff --git a/doc/formula.rst b/doc/formula.rst
index 6136466..b606aaf 100644
--- a/doc/formula.rst
+++ b/doc/formula.rst
@@ -1,56 +1,76 @@
 Formula
 *******
 
-A ``Formula`` object instance represents the chemical formula of an atom, isotope, ion, molecule, molecular ion, or certain sorts of other particle. The ``Formula`` class has methods for producing representations of the formula in HTML, LaTeX and plain text.
+A ``Formula`` object instance represents the chemical formula of an atom, isotope, ion,
+molecule, molecular ion, or certain sorts of other particle.
+The ``Formula`` class has methods for producing representations of the formula in HTML,
+LaTeX and plain text.
 
-``Formula`` objects are not supposed to be unique (different ``Formula`` objects can represent the same formula); nor is their syntax or this library designed to be applied to large or complex molecules. PyValem is intended as a lightweight, easy-to-use library with an expressive syntax for representing many common small atoms and molecules and their isotopes, states and reactions.
+``Formula`` objects are not supposed to be unique (different ``Formula`` objects can
+represent the same formula); nor is their syntax or this library designed to be applied
+to large or complex molecules.
+PyValem is intended as a lightweight, easy-to-use library with an expressive syntax
+for representing many common small atoms and molecules and their isotopes,
+states and reactions.
 
-Furthermore, whilst some validation functionality is built into the library, PyValem does not attempt to verify that a provided formula is chemically plausible. In particular, it knows nothing about valence or oxidation state.
+Furthermore, whilst some validation functionality is built into the library,
+PyValem does not attempt to verify that a provided formula is chemically plausible.
+In particular, it knows nothing about valence or oxidation state.
 
 
 Instantiation
 =============
 
-A ``Formula`` object may be instantiated by passing a valid string, conforming to the following grammar:
-
-* Single atoms, with atomic weights given by a default natural isotopic abundance are specfied with their element symbol, e.g. ``H``, ``Be``, ``Fr``.
-
-* Isotopes are specified in parentheses (round brackets) with the isotope mass number preceding the element symbol, e.g. ``(12C)``, ``(35Cl)``, ``(235U)``. Note that no caret (``^``) is used to indicate a superscript.
-
-* Charged species are specified with the charge following the formula in the format ``+n`` or ``-n``, where ``n`` may be omitted if it is 1. Do not use a caret (``^``) to indicate a superscript. For example, ``He+``, ``C+2``, ``W-``, ``(79Br)-2``.
-
-* Molecular formulas are written as a sequence of element symbols (which may be repeated for clarity over the structure), with their stoichiometries specified as an integer following the symbol. No underscore (``_``) character is used. For example, ``H2O``, ``(1H)2(16O)``, ``C2H6OH``, ``CH3CH2OH``, ``NH+``, ``CO3+2``.
-
+A ``Formula`` object may be instantiated by passing a valid string, conforming to the
+following grammar:
+
+* Single atoms, with atomic weights given by a default natural isotopic abundance are
+  specified with their element symbol, e.g. ``H``, ``Be``, ``Fr``.
+* Isotopes are specified in parentheses (round brackets) with the isotope mass number
+  preceding the element symbol, e.g. ``(12C)``, ``(35Cl)``, ``(235U)``.
+  Note that no caret (``^``) is used to indicate a superscript.
+* Charged species are specified with the charge following the formula in the format
+  ``+n`` or ``-n``, where ``n`` may be omitted if it is 1.
+  Do not use a caret (``^``) to indicate a superscript.
+  For example, ``He+``, ``C+2``, ``W-``, ``(79Br)-2``.
+* Molecular formulas are written as a sequence of element symbols (which may be
+  repeated for clarity over the structure), with their stoichiometries specified as
+  an integer following the symbol. No underscore (``_``) character is used.
+  For example, ``H2O``, ``(1H)2(16O)``, ``C2H6OH``, ``CH3CH2OH``, ``NH+``, ``CO3+2``.
 * Moieties within formula can be bracketed for clarity, for example ``CH3C(CH3)2CH3``.
-
-* A limited number of formula prefixes are supported, for example ``L-CH3CH(NH2)CO2H``, ``cis-CH3CHCHCH3``, ``ortho-C6H4(CH3)2``
-
+* A limited number of formula prefixes are supported, for example ``L-CH3CH(NH2)CO2H``,
+  ``cis-CH3CHCHCH3``, ``ortho-C6H4(CH3)2``
 * There are some special species:
     * ``e-`` is the electron;
     * ``e+`` is the positron;
-    * ``M`` is a generic third-body with no specific identity (and no defined mass or charge);
+    * ``M`` is a generic third-body with no specific identity (and no defined mass or
+      charge);
     * ``hν`` (or ``hv``) is the photon.
 
 
 Output as HTML, LaTeX and slugs
 ===============================
 
-The ``Formula`` attributes ``html`` and ``latex`` return strings representing the formula in HTML and LaTeX respectively. The attribute ``slug`` returns a URL-safe slug which uniquely identifies the formula's plain text string. For example::
+The ``Formula`` attributes ``html`` and ``latex`` return strings representing the
+formula in HTML and LaTeX respectively.
+The attribute ``slug`` returns a URL-safe slug which uniquely identifies the
+formula's plain text string. For example:
 
-    In [1]: from pyvalem.formula import Formula
+.. code-block:: pycon
 
-    In [2]: f = Formula('')
-    In [3]: print(f.formula)      # or simply print(f)
+    >>> from pyvalem.formula import Formula
+    >>> f = Formula("Co(H2O)6+2")
+    >>> print(f.formula)  # or simply print(f)
     Co(H2O)6+2
 
-    In [4]: print(f.html)
+    >>> print(f.html)
     Co(H<sub>2</sub>O)<sub>6</sub><sup>2+</sup>
 
-    In [5]: print(f.latex)
+    >>> print(f.latex)
     \mathrm{Co}(\mathrm{H}_{2}\mathrm{O})_{6}^{2+}
 
-    In [6]: print(f.slug)
-    Co-_l_H2O_r_6_p2    
+    >>> print(f.slug)
+    Co-_l_H2O_r_6_p2
 
 The HTML and LaTeX representations render as:
 
@@ -66,25 +86,32 @@ The HTML and LaTeX representations render as:
 Formula Attributes
 ==================
 
-``Formula`` objects can count atoms, calculate masses and record the total species charge::
+``Formula`` objects can count atoms, calculate masses, record the total species charge,
+etc:
+
+.. code-block:: pycon
 
-    In [7]: f = Formula('CO3-2')
-    In [8]: print(f.natoms)
+    >>> f = Formula('CO3-2')
+    >>> f.natoms
     4
-    In [9]: print(f.rmm)
+
+    >>> round(f.rmm, 3)  # relative molecular mass
     60.008
 
-    In [10]: print(f.charge)
+    >>> f.charge
     -2
 
-    In [11]: lys = Formula('(NH3+)(CH2)4CH(NH2)CO2-')
-    In [12]: print(lys.natoms)
+    >>> dict(f.atom_stoich)
+    {'C': 1, 'O': 3}
+
+    >>> lys = Formula('(NH3+)(CH2)4CH(NH2)CO2-')
+    >>> lys.natoms
     24
 
-    In [13]: print(lys.rmm)    # relative molecular mass
+    >>> round(lys.rmm, 2)
     146.19
 
-    In [14]: print(lys.charge)
+    >>> lys.charge
     0
 
 This last example is the Lysine zwitterion,
@@ -97,16 +124,22 @@ This last example is the Lysine zwitterion,
 
     $(\mathrm{N}\mathrm{H}_{3}^{+})(\mathrm{C}\mathrm{H}_{2})_{4}\mathrm{C}\mathrm{H}(\mathrm{N}\mathrm{H}_{2})\mathrm{C}\mathrm{O}_{2}^{-}$
 
-The same applies to isotopes and isotopically-pure molecules, in which case the exact mass is held by the ``mass`` attribute::
+The same applies to isotopes and isotopically-pure molecules, in which case the exact
+mass is held by the ``mass`` attribute:
+
+.. code-block:: pycon
 
-    In [15]: f = formula('(1H)(35Cl)+')
-    In [16]: print(f.mass)
+    >>> f = Formula('(1H)(35Cl)+')
+    >>> f.mass
     35.9766777262
 
-The stoichiometric formula can be output either in order of increasing atomic number (the default) or in alphabetical order::
+The stoichiometric formula can be output either in order of increasing atomic number
+(the default) or in alphabetical order:
+
+.. code-block:: pycon
 
-    In [17]: print(lys.stoichiometric_formula())
-    H14C6N2O2
+    >>> lys.stoichiometric_formula()
+    'H14C6N2O2'
 
-    In [18]: print(lys.stoichiometric_formula('alphabetical'))
-    C6H14N2O2
+    >>> lys.stoichiometric_formula('alphabetical')
+    'C6H14N2O2'
diff --git a/pyproject.toml b/pyproject.toml
index e4e9fb5..40d5b93 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -3,7 +3,7 @@ requires = ["setuptools>=43", "wheel"]
 build-backend = "setuptools.build_meta"
 
 [tool.pytest.ini_options]
-addopts = "--doctest-modules --doctest-glob='README.rst'"
+addopts = "--doctest-modules --doctest-glob='**/*.rst'"
 
 [tool.tox]
 legacy_tox_ini = """
diff --git a/src/pyvalem/states/vibrational_state.py b/src/pyvalem/states/vibrational_state.py
index 5743002..5477c20 100644
--- a/src/pyvalem/states/vibrational_state.py
+++ b/src/pyvalem/states/vibrational_state.py
@@ -31,8 +31,6 @@ def __init__(self, n, mode):
     def __repr__(self):
         return "{}ν{:d}".format(self.n if self.n != 1 else "", self.mode)
 
-    __str__ = __repr__
-
     @property
     def html(self):
         return "{}ν<sub>{:d}</sub>".format(self.n if self.n != 1 else "", self.mode)