getHeavyAtomCount() {
    let hac = 0;

    for (var i = 0; i < this.graph.vertices.length; i++) {
      if (this.graph.vertices[i].value.element !== 'H') {
        hac++;
      }
    }

    return hac;
  }
Returns the number of heavy atoms (non-hydrogen) in the current molecule.

@returns {Number} The heavy atom count.