Prime Number anagram sorter

AnagramSorter.cs

static class Program
{
    static void Main(string[] args)
    {
        var groupedWords = new Dictionary<double, List<string>>();

        string line;
        using (var reader = new StreamReader("words_alpha.txt"))
        {
            while ((line = reader.ReadLine()) != null)
            {
                var product = line.ToPrimeMultiple();
                if (groupedWords.ContainsKey(product))
                {
                    groupedWords[product].Add(line);
                }
                else
                {
                    groupedWords[product] = new List<string>() {line};
                }
            }
        }

        Console.WriteLine(String.Join(" ", groupedWords.OrderByDescending(x => x.Value.Count).First().Value));
        Console.ReadLine();
    }

    public static double ToPrimeMultiple(this string word)
    {
        return word.Aggregate(1d, (agg, letter) => agg * Map.Single(x => x.letter == letter).prime);
    }

    private static readonly HashSet<(char letter, double prime)> Map = new HashSet<(char letter, double prime)>
    {
        ('a', 2),
        ('b', 3),
        ('c', 5),
        ('d', 7),
        ('e', 11),
        ('f', 13),
        ('g', 17),
        ('h', 19),
        ('i', 23),
        ('j', 29),
        ('k', 31),
        ('l', 37),
        ('m', 41),
        ('n', 43),
        ('o', 47),
        ('p', 53),
        ('q', 59),
        ('r', 61),
        ('s', 67),
        ('t', 71),
        ('u', 73),
        ('v', 79),
        ('w', 83),
        ('x', 89),
        ('y', 97),
        ('z', 101)
    };
}